---
title: "Dockerfile reference"
description: "Dockerfiles use a simple DSL which allows you to automate the steps you would normally manually take to create an image."
keywords: "builder, docker, Dockerfile, automation, image creation"
---

<!-- This file is maintained within the docker/docker Github
     repository at https://github.com/docker/docker/. Make all
     pull requests against that repo. If you see this file in
     another repository, consider it read-only there, as it will
     periodically be overwritten by the definitive file. Pull
     requests which include edits to this file in other repositories
     will be rejected.
-->

# Dockerfile reference

Docker can build images automatically by reading the instructions from a
`Dockerfile`. A `Dockerfile` is a text document that contains all the commands a
user could call on the command line to assemble an image. Using `docker build`
users can create an automated build that executes several command-line
instructions in succession.

This page describes the commands you can use in a `Dockerfile`. When you are
done reading this page, refer to the [`Dockerfile` Best
Practices](https://docs.docker.com/engine/userguide/eng-image/dockerfile_best-practices/) for a tip-oriented guide.

## Usage

The [`docker build`](commandline/build.md) command builds an image from
a `Dockerfile` and a *context*. The build's context is the files at a specified
location `PATH` or `URL`. The `PATH` is a directory on your local filesystem.
The `URL` is a Git repository location.

A context is processed recursively. So, a `PATH` includes any subdirectories and
the `URL` includes the repository and its submodules. A simple build command
that uses the current directory as context:

    $ docker build .
    Sending build context to Docker daemon  6.51 MB
    ...

The build is run by the Docker daemon, not by the CLI. The first thing a build
process does is send the entire context (recursively) to the daemon.  In most
cases, it's best to start with an empty directory as context and keep your
Dockerfile in that directory. Add only the files needed for building the
Dockerfile.

>**Warning**: Do not use your root directory, `/`, as the `PATH` as it causes
>the build to transfer the entire contents of your hard drive to the Docker
>daemon.

To use a file in the build context, the `Dockerfile` refers to the file specified
in an instruction, for example,  a `COPY` instruction. To increase the build's
performance, exclude files and directories by adding a `.dockerignore` file to
the context directory.  For information about how to [create a `.dockerignore`
file](#dockerignore-file) see the documentation on this page.

Traditionally, the `Dockerfile` is called `Dockerfile` and located in the root
of the context. You use the `-f` flag with `docker build` to point to a Dockerfile
anywhere in your file system.

    $ docker build -f /path/to/a/Dockerfile .

You can specify a repository and tag at which to save the new image if
the build succeeds:

    $ docker build -t shykes/myapp .

To tag the image into multiple repositories after the build,
add multiple `-t` parameters when you run the `build` command:

    $ docker build -t shykes/myapp:1.0.2 -t shykes/myapp:latest .

Before the Docker daemon runs the instructions in the `Dockerfile`, it performs
a preliminary validation of the `Dockerfile` and returns an error if the syntax is incorrect:

    $ docker build -t test/myapp .
    Sending build context to Docker daemon 2.048 kB
    Error response from daemon: Unknown instruction: RUNCMD

The Docker daemon runs the instructions in the `Dockerfile` one-by-one,
committing the result of each instruction
to a new image if necessary, before finally outputting the ID of your
new image. The Docker daemon will automatically clean up the context you
sent.

Note that each instruction is run independently, and causes a new image
to be created - so `RUN cd /tmp` will not have any effect on the next
instructions.

Whenever possible, Docker will re-use the intermediate images (cache),
to accelerate the `docker build` process significantly. This is indicated by
the `Using cache` message in the console output.
(For more information, see the [Build cache section](https://docs.docker.com/engine/userguide/eng-image/dockerfile_best-practices/#/build-cache)) in the
`Dockerfile` best practices guide:

    $ docker build -t svendowideit/ambassador .
    Sending build context to Docker daemon 15.36 kB
    Step 1/4 : FROM alpine:3.2
     ---> 31f630c65071
    Step 2/4 : MAINTAINER SvenDowideit@home.org.au
     ---> Using cache
     ---> 2a1c91448f5f
    Step 3/4 : RUN apk update &&      apk add socat &&        rm -r /var/cache/
     ---> Using cache
     ---> 21ed6e7fbb73
    Step 4/4 : CMD env | grep _TCP= | (sed 's/.*_PORT_\([0-9]*\)_TCP=tcp:\/\/\(.*\):\(.*\)/socat -t 100000000 TCP4-LISTEN:\1,fork,reuseaddr TCP4:\2:\3 \&/' && echo wait) | sh
     ---> Using cache
     ---> 7ea8aef582cc
    Successfully built 7ea8aef582cc

Build cache is only used from images that have a local parent chain. This means
that these images were created by previous builds or the whole chain of images
was loaded with `docker load`. If you wish to use build cache of a specific
image you can specify it with `--cache-from` option. Images specified with
`--cache-from` do not need to have a parent chain and may be pulled from other
registries.

When you're done with your build, you're ready to look into [*Pushing a
repository to its registry*](https://docs.docker.com/engine/tutorials/dockerrepos/#/contributing-to-docker-hub).

## Format

Here is the format of the `Dockerfile`:

```Dockerfile
# Comment
INSTRUCTION arguments
```

The instruction is not case-sensitive. However, convention is for them to
be UPPERCASE to distinguish them from arguments more easily.


Docker runs instructions in a `Dockerfile` in order. **The first
instruction must be \`FROM\`** in order to specify the [*Base
Image*](glossary.md#base-image) from which you are building.

Docker treats lines that *begin* with `#` as a comment, unless the line is
a valid [parser directive](#parser-directives). A `#` marker anywhere
else in a line is treated as an argument. This allows statements like:

```Dockerfile
# Comment
RUN echo 'we are running some # of cool things'
```

Line continuation characters are not supported in comments.

## Parser directives

Parser directives are optional, and affect the way in which subsequent lines
in a `Dockerfile` are handled. Parser directives do not add layers to the build,
and will not be shown as a build step. Parser directives are written as a
special type of comment in the form `# directive=value`. A single directive
may only be used once.

Once a comment, empty line or builder instruction has been processed, Docker
no longer looks for parser directives. Instead it treats anything formatted
as a parser directive as a comment and does not attempt to validate if it might
be a parser directive. Therefore, all parser directives must be at the very
top of a `Dockerfile`.

Parser directives are not case-sensitive. However, convention is for them to
be lowercase. Convention is also to include a blank line following any
parser directives. Line continuation characters are not supported in parser
directives.

Due to these rules, the following examples are all invalid:

Invalid due to line continuation:

```Dockerfile
# direc \
tive=value
```

Invalid due to appearing twice:

```Dockerfile
# directive=value1
# directive=value2

FROM ImageName
```

Treated as a comment due to appearing after a builder instruction:

```Dockerfile
FROM ImageName
# directive=value
```

Treated as a comment due to appearing after a comment which is not a parser
directive:

```Dockerfile
# About my dockerfile
FROM ImageName
# directive=value
```

The unknown directive is treated as a comment due to not being recognized. In
addition, the known directive is treated as a comment due to appearing after
a comment which is not a parser directive.

```Dockerfile
# unknowndirective=value
# knowndirective=value
```

Non line-breaking whitespace is permitted in a parser directive. Hence, the
following lines are all treated identically:

```Dockerfile
#directive=value
# directive =value
#	directive= value
# directive = value
#	  dIrEcTiVe=value
```

The following parser directive is supported:

* `escape`

## escape

    # escape=\ (backslash)

Or

    # escape=` (backtick)

The `escape` directive sets the character used to escape characters in a
`Dockerfile`. If not specified, the default escape character is `\`.

The escape character is used both to escape characters in a line, and to
escape a newline. This allows a `Dockerfile` instruction to
span multiple lines. Note that regardless of whether the `escape` parser
directive is included in a `Dockerfile`, *escaping is not performed in
a `RUN` command, except at the end of a line.*

Setting the escape character to `` ` `` is especially useful on
`Windows`, where `\` is the directory path separator. `` ` `` is consistent
with [Windows PowerShell](https://technet.microsoft.com/en-us/library/hh847755.aspx).

Consider the following example which would fail in a non-obvious way on
`Windows`. The second `\` at the end of the second line would be interpreted as an
escape for the newline, instead of a target of the escape from the first `\`.
Similarly, the `\` at the end of the third line would, assuming it was actually
handled as an instruction, cause it be treated as a line continuation. The result
of this dockerfile is that second and third lines are considered a single
instruction:

```Dockerfile
FROM microsoft/nanoserver
COPY testfile.txt c:\\
RUN dir c:\
```

Results in:

    PS C:\John> docker build -t cmd .
    Sending build context to Docker daemon 3.072 kB
    Step 1/2 : FROM microsoft/nanoserver
     ---> 22738ff49c6d
    Step 2/2 : COPY testfile.txt c:\RUN dir c:
    GetFileAttributesEx c:RUN: The system cannot find the file specified.
    PS C:\John>

One solution to the above would be to use `/` as the target of both the `COPY`
instruction, and `dir`. However, this syntax is, at best, confusing as it is not
natural for paths on `Windows`, and at worst, error prone as not all commands on
`Windows` support `/` as the path separator.

By adding the `escape` parser directive, the following `Dockerfile` succeeds as
expected with the use of natural platform semantics for file paths on `Windows`:

    # escape=`

    FROM microsoft/nanoserver
    COPY testfile.txt c:\
    RUN dir c:\

Results in:

    PS C:\John> docker build -t succeeds --no-cache=true .
    Sending build context to Docker daemon 3.072 kB
    Step 1/3 : FROM microsoft/nanoserver
     ---> 22738ff49c6d
    Step 2/3 : COPY testfile.txt c:\
     ---> 96655de338de
    Removing intermediate container 4db9acbb1682
    Step 3/3 : RUN dir c:\
     ---> Running in a2c157f842f5
     Volume in drive C has no label.
     Volume Serial Number is 7E6D-E0F7
    
     Directory of c:\
    
    10/05/2016  05:04 PM             1,894 License.txt
    10/05/2016  02:22 PM    <DIR>          Program Files
    10/05/2016  02:14 PM    <DIR>          Program Files (x86)
    10/28/2016  11:18 AM                62 testfile.txt
    10/28/2016  11:20 AM    <DIR>          Users
    10/28/2016  11:20 AM    <DIR>          Windows
               2 File(s)          1,956 bytes
               4 Dir(s)  21,259,096,064 bytes free
     ---> 01c7f3bef04f
    Removing intermediate container a2c157f842f5
    Successfully built 01c7f3bef04f
    PS C:\John>

## Environment replacement

Environment variables (declared with [the `ENV` statement](#env)) can also be
used in certain instructions as variables to be interpreted by the
`Dockerfile`. Escapes are also handled for including variable-like syntax
into a statement literally.

Environment variables are notated in the `Dockerfile` either with
`$variable_name` or `${variable_name}`. They are treated equivalently and the
brace syntax is typically used to address issues with variable names with no
whitespace, like `${foo}_bar`.

The `${variable_name}` syntax also supports a few of the standard `bash`
modifiers as specified below:

* `${variable:-word}` indicates that if `variable` is set then the result
  will be that value. If `variable` is not set then `word` will be the result.
* `${variable:+word}` indicates that if `variable` is set then `word` will be
  the result, otherwise the result is the empty string.

In all cases, `word` can be any string, including additional environment
variables.

Escaping is possible by adding a `\` before the variable: `\$foo` or `\${foo}`,
for example, will translate to `$foo` and `${foo}` literals respectively.

Example (parsed representation is displayed after the `#`):

    FROM busybox
    ENV foo /bar
    WORKDIR ${foo}   # WORKDIR /bar
    ADD . $foo       # ADD . /bar
    COPY \$foo /quux # COPY $foo /quux

Environment variables are supported by the following list of instructions in
the `Dockerfile`:

* `ADD`
* `COPY`
* `ENV`
* `EXPOSE`
* `LABEL`
* `USER`
* `WORKDIR`
* `VOLUME`
* `STOPSIGNAL`

as well as:

* `ONBUILD` (when combined with one of the supported instructions above)

> **Note**:
> prior to 1.4, `ONBUILD` instructions did **NOT** support environment
> variable, even when combined with any of the instructions listed above.

Environment variable substitution will use the same value for each variable
throughout the entire command. In other words, in this example:

    ENV abc=hello
    ENV abc=bye def=$abc
    ENV ghi=$abc

will result in `def` having a value of `hello`, not `bye`. However,
`ghi` will have a value of `bye` because it is not part of the same command
that set `abc` to `bye`.

## .dockerignore file

Before the docker CLI sends the context to the docker daemon, it looks
for a file named `.dockerignore` in the root directory of the context.
If this file exists, the CLI modifies the context to exclude files and
directories that match patterns in it.  This helps to avoid
unnecessarily sending large or sensitive files and directories to the
daemon and potentially adding them to images using `ADD` or `COPY`.

The CLI interprets the `.dockerignore` file as a newline-separated
list of patterns similar to the file globs of Unix shells.  For the
purposes of matching, the root of the context is considered to be both
the working and the root directory.  For example, the patterns
`/foo/bar` and `foo/bar` both exclude a file or directory named `bar`
in the `foo` subdirectory of `PATH` or in the root of the git
repository located at `URL`.  Neither excludes anything else.

If a line in `.dockerignore` file starts with `#` in column 1, then this line is
considered as a comment and is ignored before interpreted by the CLI.

Here is an example `.dockerignore` file:

```
# comment
    */temp*
    */*/temp*
    temp?
```

This file causes the following build behavior:

| Rule           | Behavior                                                                                                                                                                     |
|----------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| `# comment`    | Ignored.                 |
| `*/temp*`      | Exclude files and directories whose names start with `temp` in any immediate subdirectory of the root.  For example, the plain file `/somedir/temporary.txt` is excluded, as is the directory `/somedir/temp`.                 |
| `*/*/temp*`    | Exclude files and directories starting with `temp` from any subdirectory that is two levels below the root. For example, `/somedir/subdir/temporary.txt` is excluded. |
| `temp?`        | Exclude files and directories in the root directory whose names are a one-character extension of `temp`.  For example, `/tempa` and `/tempb` are excluded.


Matching is done using Go's
[filepath.Match](http://golang.org/pkg/path/filepath#Match) rules.  A
preprocessing step removes leading and trailing whitespace and
eliminates `.` and `..` elements using Go's
[filepath.Clean](http://golang.org/pkg/path/filepath/#Clean).  Lines
that are blank after preprocessing are ignored.

Beyond Go's filepath.Match rules, Docker also supports a special
wildcard string `**` that matches any number of directories (including
zero). For example, `**/*.go` will exclude all files that end with `.go`
that are found in all directories, including the root of the build context.

Lines starting with `!` (exclamation mark) can be used to make exceptions
to exclusions.  The following is an example `.dockerignore` file that
uses this mechanism:

```
    *.md
    !README.md
```

All markdown files *except* `README.md` are excluded from the context.

The placement of `!` exception rules influences the behavior: the last
line of the `.dockerignore` that matches a particular file determines
whether it is included or excluded.  Consider the following example:

```
    *.md
    !README*.md
    README-secret.md
```

No markdown files are included in the context except README files other than
`README-secret.md`.

Now consider this example:

```
    *.md
    README-secret.md
    !README*.md
```

All of the README files are included.  The middle line has no effect because
`!README*.md` matches `README-secret.md` and comes last.

You can even use the `.dockerignore` file to exclude the `Dockerfile`
and `.dockerignore` files.  These files are still sent to the daemon
because it needs them to do its job.  But the `ADD` and `COPY` commands
do not copy them to the image.

Finally, you may want to specify which files to include in the
context, rather than which to exclude. To achieve this, specify `*` as
the first pattern, followed by one or more `!` exception patterns.

**Note**: For historical reasons, the pattern `.` is ignored.

## FROM

    FROM <image>

Or

    FROM <image>:<tag>

Or

    FROM <image>@<digest>

The `FROM` instruction sets the [*Base Image*](glossary.md#base-image)
for subsequent instructions. As such, a valid `Dockerfile` must have `FROM` as
its first instruction. The image can be any valid image – it is especially easy
to start by **pulling an image** from the [*Public Repositories*](https://docs.docker.com/engine/tutorials/dockerrepos/).

- `FROM` must be the first non-comment instruction in the `Dockerfile`.

- `FROM` can appear multiple times within a single `Dockerfile` in order to create
multiple images. Simply make a note of the last image ID output by the commit
before each new `FROM` command.

- The `tag` or `digest` values are optional. If you omit either of them, the builder
assumes a `latest` by default. The builder returns an error if it cannot match
the `tag` value.

## RUN

RUN has 2 forms:

- `RUN <command>` (*shell* form, the command is run in a shell, which by
default is `/bin/sh -c` on Linux or `cmd /S /C` on Windows)
- `RUN ["executable", "param1", "param2"]` (*exec* form)

The `RUN` instruction will execute any commands in a new layer on top of the
current image and commit the results. The resulting committed image will be
used for the next step in the `Dockerfile`.

Layering `RUN` instructions and generating commits conforms to the core
concepts of Docker where commits are cheap and containers can be created from
any point in an image's history, much like source control.

The *exec* form makes it possible to avoid shell string munging, and to `RUN`
commands using a base image that does not contain the specified shell executable.

The default shell for the *shell* form can be changed using the `SHELL`
command.

In the *shell* form you can use a `\` (backslash) to continue a single
RUN instruction onto the next line. For example, consider these two lines:

```
RUN /bin/bash -c 'source $HOME/.bashrc; \
echo $HOME'
```
Together they are equivalent to this single line:

```
RUN /bin/bash -c 'source $HOME/.bashrc; echo $HOME'
```

> **Note**:
> To use a different shell, other than '/bin/sh', use the *exec* form
> passing in the desired shell. For example,
> `RUN ["/bin/bash", "-c", "echo hello"]`

> **Note**:
> The *exec* form is parsed as a JSON array, which means that
> you must use double-quotes (") around words not single-quotes (').

> **Note**:
> Unlike the *shell* form, the *exec* form does not invoke a command shell.
> This means that normal shell processing does not happen. For example,
> `RUN [ "echo", "$HOME" ]` will not do variable substitution on `$HOME`.
> If you want shell processing then either use the *shell* form or execute
> a shell directly, for example: `RUN [ "sh", "-c", "echo $HOME" ]`.
> When using the exec form and executing a shell directly, as in the case for
> the shell form, it is the shell that is doing the environment variable
> expansion, not docker.
>
> **Note**:
> In the *JSON* form, it is necessary to escape backslashes. This is
> particularly relevant on Windows where the backslash is the path separator.
> The following line would otherwise be treated as *shell* form due to not
> being valid JSON, and fail in an unexpected way:
> `RUN ["c:\windows\system32\tasklist.exe"]`
> The correct syntax for this example is:
> `RUN ["c:\\windows\\system32\\tasklist.exe"]`

The cache for `RUN` instructions isn't invalidated automatically during
the next build. The cache for an instruction like
`RUN apt-get dist-upgrade -y` will be reused during the next build. The
cache for `RUN` instructions can be invalidated by using the `--no-cache`
flag, for example `docker build --no-cache`.

See the [`Dockerfile` Best Practices
guide](https://docs.docker.com/engine/userguide/eng-image/dockerfile_best-practices/#/build-cache) for more information.

The cache for `RUN` instructions can be invalidated by `ADD` instructions. See
[below](#add) for details.

### Known issues (RUN)

- [Issue 783](https://github.com/docker/docker/issues/783) is about file
  permissions problems that can occur when using the AUFS file system. You
  might notice it during an attempt to `rm` a file, for example.

  For systems that have recent aufs version (i.e., `dirperm1` mount option can
  be set), docker will attempt to fix the issue automatically by mounting
  the layers with `dirperm1` option. More details on `dirperm1` option can be
  found at [`aufs` man page](https://github.com/sfjro/aufs3-linux/tree/aufs3.18/Documentation/filesystems/aufs)

  If your system doesn't have support for `dirperm1`, the issue describes a workaround.

## CMD

The `CMD` instruction has three forms:

- `CMD ["executable","param1","param2"]` (*exec* form, this is the preferred form)
- `CMD ["param1","param2"]` (as *default parameters to ENTRYPOINT*)
- `CMD command param1 param2` (*shell* form)

There can only be one `CMD` instruction in a `Dockerfile`. If you list more than one `CMD`
then only the last `CMD` will take effect.

**The main purpose of a `CMD` is to provide defaults for an executing
container.** These defaults can include an executable, or they can omit
the executable, in which case you must specify an `ENTRYPOINT`
instruction as well.

> **Note**:
> If `CMD` is used to provide default arguments for the `ENTRYPOINT`
> instruction, both the `CMD` and `ENTRYPOINT` instructions should be specified
> with the JSON array format.

> **Note**:
> The *exec* form is parsed as a JSON array, which means that
> you must use double-quotes (") around words not single-quotes (').

> **Note**:
> Unlike the *shell* form, the *exec* form does not invoke a command shell.
> This means that normal shell processing does not happen. For example,
> `CMD [ "echo", "$HOME" ]` will not do variable substitution on `$HOME`.
> If you want shell processing then either use the *shell* form or execute
> a shell directly, for example: `CMD [ "sh", "-c", "echo $HOME" ]`.
> When using the exec form and executing a shell directly, as in the case for
> the shell form, it is the shell that is doing the environment variable
> expansion, not docker.

When used in the shell or exec formats, the `CMD` instruction sets the command
to be executed when running the image.

If you use the *shell* form of the `CMD`, then the `<command>` will execute in
`/bin/sh -c`:

    FROM ubuntu
    CMD echo "This is a test." | wc -

If you want to **run your** `<command>` **without a shell** then you must
express the command as a JSON array and give the full path to the executable.
**This array form is the preferred format of `CMD`.** Any additional parameters
must be individually expressed as strings in the array:

    FROM ubuntu
    CMD ["/usr/bin/wc","--help"]

If you would like your container to run the same executable every time, then
you should consider using `ENTRYPOINT` in combination with `CMD`. See
[*ENTRYPOINT*](#entrypoint).

If the user specifies arguments to `docker run` then they will override the
default specified in `CMD`.

> **Note**:
> Don't confuse `RUN` with `CMD`. `RUN` actually runs a command and commits
> the result; `CMD` does not execute anything at build time, but specifies
> the intended command for the image.

## LABEL

    LABEL <key>=<value> <key>=<value> <key>=<value> ...

The `LABEL` instruction adds metadata to an image. A `LABEL` is a
key-value pair. To include spaces within a `LABEL` value, use quotes and
backslashes as you would in command-line parsing. A few usage examples:

    LABEL "com.example.vendor"="ACME Incorporated"
    LABEL com.example.label-with-value="foo"
    LABEL version="1.0"
    LABEL description="This text illustrates \
    that label-values can span multiple lines."

An image can have more than one label. To specify multiple labels,
Docker recommends combining labels into a single `LABEL` instruction where
possible. Each `LABEL` instruction produces a new layer which can result in an
inefficient image if you use many labels. This example results in a single image
layer.

    LABEL multi.label1="value1" multi.label2="value2" other="value3"

The above can also be written as:

    LABEL multi.label1="value1" \
          multi.label2="value2" \
          other="value3"

Labels are additive including `LABEL`s in `FROM` images. If Docker
encounters a label/key that already exists, the new value overrides any previous
labels with identical keys.

To view an image's labels, use the `docker inspect` command.

    "Labels": {
        "com.example.vendor": "ACME Incorporated"
        "com.example.label-with-value": "foo",
        "version": "1.0",
        "description": "This text illustrates that label-values can span multiple lines.",
        "multi.label1": "value1",
        "multi.label2": "value2",
        "other": "value3"
    },

## MAINTAINER (deprecated)

    MAINTAINER <name>

The `MAINTAINER` instruction sets the *Author* field of the generated images.
The `LABEL` instruction is a much more flexible version of this and you should use
it instead, as it enables setting any metadata you require, and can be viewed
easily, for example with `docker inspect`. To set a label corresponding to the
`MAINTAINER` field you could use:

    LABEL maintainer "SvenDowideit@home.org.au"

This will then be visible from `docker inspect` with the other labels.

## EXPOSE

    EXPOSE <port> [<port>...]

The `EXPOSE` instruction informs Docker that the container listens on the
specified network ports at runtime. `EXPOSE` does not make the ports of the
container accessible to the host. To do that, you must use either the `-p` flag
to publish a range of ports or the `-P` flag to publish all of the exposed
ports. You can expose one port number and publish it externally under another
number.

To set up port redirection on the host system, see [using the -P
flag](run.md#expose-incoming-ports). The Docker network feature supports
creating networks without the need to expose ports within the network, for
detailed information see the  [overview of this
feature](https://docs.docker.com/engine/userguide/networking/)).

## ENV

    ENV <key> <value>
    ENV <key>=<value> ...

The `ENV` instruction sets the environment variable `<key>` to the value
`<value>`. This value will be in the environment of all "descendant"
`Dockerfile` commands and can be [replaced inline](#environment-replacement) in
many as well.

The `ENV` instruction has two forms. The first form, `ENV <key> <value>`,
will set a single variable to a value. The entire string after the first
space will be treated as the `<value>` - including characters such as
spaces and quotes.

The second form, `ENV <key>=<value> ...`, allows for multiple variables to
be set at one time. Notice that the second form uses the equals sign (=)
in the syntax, while the first form does not. Like command line parsing,
quotes and backslashes can be used to include spaces within values.

For example:

    ENV myName="John Doe" myDog=Rex\ The\ Dog \
        myCat=fluffy

and

    ENV myName John Doe
    ENV myDog Rex The Dog
    ENV myCat fluffy

will yield the same net results in the final image, but the first form
is preferred because it produces a single cache layer.

The environment variables set using `ENV` will persist when a container is run
from the resulting image. You can view the values using `docker inspect`, and
change them using `docker run --env <key>=<value>`.

> **Note**:
> Environment persistence can cause unexpected side effects. For example,
> setting `ENV DEBIAN_FRONTEND noninteractive` may confuse apt-get
> users on a Debian-based image. To set a value for a single command, use
> `RUN <key>=<value> <command>`.

## ADD

ADD has two forms:

- `ADD <src>... <dest>`
- `ADD ["<src>",... "<dest>"]` (this form is required for paths containing
whitespace)

The `ADD` instruction copies new files, directories or remote file URLs from `<src>`
and adds them to the filesystem of the image at the path `<dest>`.

Multiple `<src>` resource may be specified but if they are files or
directories then they must be relative to the source directory that is
being built (the context of the build).

Each `<src>` may contain wildcards and matching will be done using Go's
[filepath.Match](http://golang.org/pkg/path/filepath#Match) rules. For example:

    ADD hom* /mydir/        # adds all files starting with "hom"
    ADD hom?.txt /mydir/    # ? is replaced with any single character, e.g., "home.txt"

The `<dest>` is an absolute path, or a path relative to `WORKDIR`, into which
the source will be copied inside the destination container.

    ADD test relativeDir/          # adds "test" to `WORKDIR`/relativeDir/
    ADD test /absoluteDir/         # adds "test" to /absoluteDir/

All new files and directories are created with a UID and GID of 0.

In the case where `<src>` is a remote file URL, the destination will
have permissions of 600. If the remote file being retrieved has an HTTP
`Last-Modified` header, the timestamp from that header will be used
to set the `mtime` on the destination file. However, like any other file
processed during an `ADD`, `mtime` will not be included in the determination
of whether or not the file has changed and the cache should be updated.

> **Note**:
> If you build by passing a `Dockerfile` through STDIN (`docker
> build - < somefile`), there is no build context, so the `Dockerfile`
> can only contain a URL based `ADD` instruction. You can also pass a
> compressed archive through STDIN: (`docker build - < archive.tar.gz`),
> the `Dockerfile` at the root of the archive and the rest of the
> archive will be used as the context of the build.

> **Note**:
> If your URL files are protected using authentication, you
> will need to use `RUN wget`, `RUN curl` or use another tool from
> within the container as the `ADD` instruction does not support
> authentication.

> **Note**:
> The first encountered `ADD` instruction will invalidate the cache for all
> following instructions from the Dockerfile if the contents of `<src>` have
> changed. This includes invalidating the cache for `RUN` instructions.
> See the [`Dockerfile` Best Practices
guide](https://docs.docker.com/engine/userguide/eng-image/dockerfile_best-practices/#/build-cache) for more information.


`ADD` obeys the following rules:

- The `<src>` path must be inside the *context* of the build;
  you cannot `ADD ../something /something`, because the first step of a
  `docker build` is to send the context directory (and subdirectories) to the
  docker daemon.

- If `<src>` is a URL and `<dest>` does not end with a trailing slash, then a
  file is downloaded from the URL and copied to `<dest>`.

- If `<src>` is a URL and `<dest>` does end with a trailing slash, then the
  filename is inferred from the URL and the file is downloaded to
  `<dest>/<filename>`. For instance, `ADD http://example.com/foobar /` would
  create the file `/foobar`. The URL must have a nontrivial path so that an
  appropriate filename can be discovered in this case (`http://example.com`
  will not work).

- If `<src>` is a directory, the entire contents of the directory are copied,
  including filesystem metadata.

> **Note**:
> The directory itself is not copied, just its contents.

- If `<src>` is a *local* tar archive in a recognized compression format
  (identity, gzip, bzip2 or xz) then it is unpacked as a directory. Resources
  from *remote* URLs are **not** decompressed. When a directory is copied or
  unpacked, it has the same behavior as `tar -x`, the result is the union of:

    1. Whatever existed at the destination path and
    2. The contents of the source tree, with conflicts resolved in favor
       of "2." on a file-by-file basis.

  > **Note**:
  > Whether a file is identified as a recognized compression format or not
  > is done solely based on the contents of the file, not the name of the file.
  > For example, if an empty file happens to end with `.tar.gz` this will not
  > be recognized as a compressed file and **will not** generate any kind of
  > decompression error message, rather the file will simply be copied to the
  > destination.

- If `<src>` is any other kind of file, it is copied individually along with
  its metadata. In this case, if `<dest>` ends with a trailing slash `/`, it
  will be considered a directory and the contents of `<src>` will be written
  at `<dest>/base(<src>)`.

- If multiple `<src>` resources are specified, either directly or due to the
  use of a wildcard, then `<dest>` must be a directory, and it must end with
  a slash `/`.

- If `<dest>` does not end with a trailing slash, it will be considered a
  regular file and the contents of `<src>` will be written at `<dest>`.

- If `<dest>` doesn't exist, it is created along with all missing directories
  in its path.

## COPY

COPY has two forms:

- `COPY <src>... <dest>`
- `COPY ["<src>",... "<dest>"]` (this form is required for paths containing
whitespace)

The `COPY` instruction copies new files or directories from `<src>`
and adds them to the filesystem of the container at the path `<dest>`.

Multiple `<src>` resource may be specified but they must be relative
to the source directory that is being built (the context of the build).

Each `<src>` may contain wildcards and matching will be done using Go's
[filepath.Match](http://golang.org/pkg/path/filepath#Match) rules. For example:

    COPY hom* /mydir/        # adds all files starting with "hom"
    COPY hom?.txt /mydir/    # ? is replaced with any single character, e.g., "home.txt"

The `<dest>` is an absolute path, or a path relative to `WORKDIR`, into which
the source will be copied inside the destination container.

    COPY test relativeDir/   # adds "test" to `WORKDIR`/relativeDir/
    COPY test /absoluteDir/  # adds "test" to /absoluteDir/

All new files and directories are created with a UID and GID of 0.

> **Note**:
> If you build using STDIN (`docker build - < somefile`), there is no
> build context, so `COPY` can't be used.

`COPY` obeys the following rules:

- The `<src>` path must be inside the *context* of the build;
  you cannot `COPY ../something /something`, because the first step of a
  `docker build` is to send the context directory (and subdirectories) to the
  docker daemon.

- If `<src>` is a directory, the entire contents of the directory are copied,
  including filesystem metadata.

> **Note**:
> The directory itself is not copied, just its contents.

- If `<src>` is any other kind of file, it is copied individually along with
  its metadata. In this case, if `<dest>` ends with a trailing slash `/`, it
  will be considered a directory and the contents of `<src>` will be written
  at `<dest>/base(<src>)`.

- If multiple `<src>` resources are specified, either directly or due to the
  use of a wildcard, then `<dest>` must be a directory, and it must end with
  a slash `/`.

- If `<dest>` does not end with a trailing slash, it will be considered a
  regular file and the contents of `<src>` will be written at `<dest>`.

- If `<dest>` doesn't exist, it is created along with all missing directories
  in its path.

## ENTRYPOINT

ENTRYPOINT has two forms:

- `ENTRYPOINT ["executable", "param1", "param2"]`
  (*exec* form, preferred)
- `ENTRYPOINT command param1 param2`
  (*shell* form)

An `ENTRYPOINT` allows you to configure a container that will run as an executable.

For example, the following will start nginx with its default content, listening
on port 80:

    docker run -i -t --rm -p 80:80 nginx

Command line arguments to `docker run <image>` will be appended after all
elements in an *exec* form `ENTRYPOINT`, and will override all elements specified
using `CMD`.
This allows arguments to be passed to the entry point, i.e., `docker run <image> -d`
will pass the `-d` argument to the entry point.
You can override the `ENTRYPOINT` instruction using the `docker run --entrypoint`
flag.

The *shell* form prevents any `CMD` or `run` command line arguments from being
used, but has the disadvantage that your `ENTRYPOINT` will be started as a
subcommand of `/bin/sh -c`, which does not pass signals.
This means that the executable will not be the container's `PID 1` - and
will _not_ receive Unix signals - so your executable will not receive a
`SIGTERM` from `docker stop <container>`.

Only the last `ENTRYPOINT` instruction in the `Dockerfile` will have an effect.

### Exec form ENTRYPOINT example

You can use the *exec* form of `ENTRYPOINT` to set fairly stable default commands
and arguments and then use either form of `CMD` to set additional defaults that
are more likely to be changed.

    FROM ubuntu
    ENTRYPOINT ["top", "-b"]
    CMD ["-c"]

When you run the container, you can see that `top` is the only process:

    $ docker run -it --rm --name test  top -H
    top - 08:25:00 up  7:27,  0 users,  load average: 0.00, 0.01, 0.05
    Threads:   1 total,   1 running,   0 sleeping,   0 stopped,   0 zombie
    %Cpu(s):  0.1 us,  0.1 sy,  0.0 ni, 99.7 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
    KiB Mem:   2056668 total,  1616832 used,   439836 free,    99352 buffers
    KiB Swap:  1441840 total,        0 used,  1441840 free.  1324440 cached Mem

      PID USER      PR  NI    VIRT    RES    SHR S %CPU %MEM     TIME+ COMMAND
        1 root      20   0   19744   2336   2080 R  0.0  0.1   0:00.04 top

To examine the result further, you can use `docker exec`:

    $ docker exec -it test ps aux
    USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
    root         1  2.6  0.1  19752  2352 ?        Ss+  08:24   0:00 top -b -H
    root         7  0.0  0.1  15572  2164 ?        R+   08:25   0:00 ps aux

And you can gracefully request `top` to shut down using `docker stop test`.

The following `Dockerfile` shows using the `ENTRYPOINT` to run Apache in the
foreground (i.e., as `PID 1`):

```
FROM debian:stable
RUN apt-get update && apt-get install -y --force-yes apache2
EXPOSE 80 443
VOLUME ["/var/www", "/var/log/apache2", "/etc/apache2"]
ENTRYPOINT ["/usr/sbin/apache2ctl", "-D", "FOREGROUND"]
```

If you need to write a starter script for a single executable, you can ensure that
the final executable receives the Unix signals by using `exec` and `gosu`
commands:

```bash
#!/bin/bash
set -e

if [ "$1" = 'postgres' ]; then
    chown -R postgres "$PGDATA"

    if [ -z "$(ls -A "$PGDATA")" ]; then
        gosu postgres initdb
    fi

    exec gosu postgres "$@"
fi

exec "$@"
```

Lastly, if you need to do some extra cleanup (or communicate with other containers)
on shutdown, or are co-ordinating more than one executable, you may need to ensure
that the `ENTRYPOINT` script receives the Unix signals, passes them on, and then
does some more work:

```
#!/bin/sh
# Note: I've written this using sh so it works in the busybox container too

# USE the trap if you need to also do manual cleanup after the service is stopped,
#     or need to start multiple services in the one container
trap "echo TRAPed signal" HUP INT QUIT TERM

# start service in background here
/usr/sbin/apachectl start

echo "[hit enter key to exit] or run 'docker stop <container>'"
read

# stop service and clean up here
echo "stopping apache"
/usr/sbin/apachectl stop

echo "exited $0"
```

If you run this image with `docker run -it --rm -p 80:80 --name test apache`,
you can then examine the container's processes with `docker exec`, or `docker top`,
and then ask the script to stop Apache:

```bash
$ docker exec -it test ps aux
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.1  0.0   4448   692 ?        Ss+  00:42   0:00 /bin/sh /run.sh 123 cmd cmd2
root        19  0.0  0.2  71304  4440 ?        Ss   00:42   0:00 /usr/sbin/apache2 -k start
www-data    20  0.2  0.2 360468  6004 ?        Sl   00:42   0:00 /usr/sbin/apache2 -k start
www-data    21  0.2  0.2 360468  6000 ?        Sl   00:42   0:00 /usr/sbin/apache2 -k start
root        81  0.0  0.1  15572  2140 ?        R+   00:44   0:00 ps aux
$ docker top test
PID                 USER                COMMAND
10035               root                {run.sh} /bin/sh /run.sh 123 cmd cmd2
10054               root                /usr/sbin/apache2 -k start
10055               33                  /usr/sbin/apache2 -k start
10056               33                  /usr/sbin/apache2 -k start
$ /usr/bin/time docker stop test
test
real	0m 0.27s
user	0m 0.03s
sys	0m 0.03s
```

> **Note:** you can override the `ENTRYPOINT` setting using `--entrypoint`,
> but this can only set the binary to *exec* (no `sh -c` will be used).

> **Note**:
> The *exec* form is parsed as a JSON array, which means that
> you must use double-quotes (") around words not single-quotes (').

> **Note**:
> Unlike the *shell* form, the *exec* form does not invoke a command shell.
> This means that normal shell processing does not happen. For example,
> `ENTRYPOINT [ "echo", "$HOME" ]` will not do variable substitution on `$HOME`.
> If you want shell processing then either use the *shell* form or execute
> a shell directly, for example: `ENTRYPOINT [ "sh", "-c", "echo $HOME" ]`.
> When using the exec form and executing a shell directly, as in the case for
> the shell form, it is the shell that is doing the environment variable
> expansion, not docker.

### Shell form ENTRYPOINT example

You can specify a plain string for the `ENTRYPOINT` and it will execute in `/bin/sh -c`.
This form will use shell processing to substitute shell environment variables,
and will ignore any `CMD` or `docker run` command line arguments.
To ensure that `docker stop` will signal any long running `ENTRYPOINT` executable
correctly, you need to remember to start it with `exec`:

    FROM ubuntu
    ENTRYPOINT exec top -b

When you run this image, you'll see the single `PID 1` process:

    $ docker run -it --rm --name test top
    Mem: 1704520K used, 352148K free, 0K shrd, 0K buff, 140368121167873K cached
    CPU:   5% usr   0% sys   0% nic  94% idle   0% io   0% irq   0% sirq
    Load average: 0.08 0.03 0.05 2/98 6
      PID  PPID USER     STAT   VSZ %VSZ %CPU COMMAND
        1     0 root     R     3164   0%   0% top -b

Which will exit cleanly on `docker stop`:

    $ /usr/bin/time docker stop test
    test
    real	0m 0.20s
    user	0m 0.02s
    sys	0m 0.04s

If you forget to add `exec` to the beginning of your `ENTRYPOINT`:

    FROM ubuntu
    ENTRYPOINT top -b
    CMD --ignored-param1

You can then run it (giving it a name for the next step):

    $ docker run -it --name test top --ignored-param2
    Mem: 1704184K used, 352484K free, 0K shrd, 0K buff, 140621524238337K cached
    CPU:   9% usr   2% sys   0% nic  88% idle   0% io   0% irq   0% sirq
    Load average: 0.01 0.02 0.05 2/101 7
      PID  PPID USER     STAT   VSZ %VSZ %CPU COMMAND
        1     0 root     S     3168   0%   0% /bin/sh -c top -b cmd cmd2
        7     1 root     R     3164   0%   0% top -b

You can see from the output of `top` that the specified `ENTRYPOINT` is not `PID 1`.

If you then run `docker stop test`, the container will not exit cleanly - the
`stop` command will be forced to send a `SIGKILL` after the timeout:

    $ docker exec -it test ps aux
    PID   USER     COMMAND
        1 root     /bin/sh -c top -b cmd cmd2
        7 root     top -b
        8 root     ps aux
    $ /usr/bin/time docker stop test
    test
    real	0m 10.19s
    user	0m 0.04s
    sys	0m 0.03s

### Understand how CMD and ENTRYPOINT interact

Both `CMD` and `ENTRYPOINT` instructions define what command gets executed when running a container.
There are few rules that describe their co-operation.

1. Dockerfile should specify at least one of `CMD` or `ENTRYPOINT` commands.

2. `ENTRYPOINT` should be defined when using the container as an executable.

3. `CMD` should be used as a way of defining default arguments for an `ENTRYPOINT` command
or for executing an ad-hoc command in a container.

4. `CMD` will be overridden when running the container with alternative arguments.

The table below shows what command is executed for different `ENTRYPOINT` / `CMD` combinations:

|                                | No ENTRYPOINT              | ENTRYPOINT exec_entry p1_entry | ENTRYPOINT ["exec_entry", "p1_entry"]          |
|--------------------------------|----------------------------|--------------------------------|------------------------------------------------|
| **No CMD**                     | *error, not allowed*       | /bin/sh -c exec_entry p1_entry | exec_entry p1_entry                            |
| **CMD ["exec_cmd", "p1_cmd"]** | exec_cmd p1_cmd            | /bin/sh -c exec_entry p1_entry | exec_entry p1_entry exec_cmd p1_cmd            |
| **CMD ["p1_cmd", "p2_cmd"]**   | p1_cmd p2_cmd              | /bin/sh -c exec_entry p1_entry | exec_entry p1_entry p1_cmd p2_cmd              |
| **CMD exec_cmd p1_cmd**        | /bin/sh -c exec_cmd p1_cmd | /bin/sh -c exec_entry p1_entry | exec_entry p1_entry /bin/sh -c exec_cmd p1_cmd |

## VOLUME

    VOLUME ["/data"]

The `VOLUME` instruction creates a mount point with the specified name
and marks it as holding externally mounted volumes from native host or other
containers. The value can be a JSON array, `VOLUME ["/var/log/"]`, or a plain
string with multiple arguments, such as `VOLUME /var/log` or `VOLUME /var/log
/var/db`. For more information/examples and mounting instructions via the
Docker client, refer to
[*Share Directories via Volumes*](https://docs.docker.com/engine/tutorials/dockervolumes/#/mount-a-host-directory-as-a-data-volume)
documentation.

The `docker run` command initializes the newly created volume with any data
that exists at the specified location within the base image. For example,
consider the following Dockerfile snippet:

    FROM ubuntu
    RUN mkdir /myvol
    RUN echo "hello world" > /myvol/greeting
    VOLUME /myvol

This Dockerfile results in an image that causes `docker run`, to
create a new mount point at `/myvol` and copy the  `greeting` file
into the newly created volume.

> **Note**:
> If any build steps change the data within the volume after it has been
> declared, those changes will be discarded.

> **Note**:
> The list is parsed as a JSON array, which means that
> you must use double-quotes (") around words not single-quotes (').

## USER

    USER daemon

The `USER` instruction sets the user name or UID to use when running the image
and for any `RUN`, `CMD` and `ENTRYPOINT` instructions that follow it in the
`Dockerfile`.

## WORKDIR

    WORKDIR /path/to/workdir

The `WORKDIR` instruction sets the working directory for any `RUN`, `CMD`,
`ENTRYPOINT`, `COPY` and `ADD` instructions that follow it in the `Dockerfile`.
If the `WORKDIR` doesn't exist, it will be created even if it's not used in any
subsequent `Dockerfile` instruction.

It can be used multiple times in the one `Dockerfile`. If a relative path
is provided, it will be relative to the path of the previous `WORKDIR`
instruction. For example:

    WORKDIR /a
    WORKDIR b
    WORKDIR c
    RUN pwd

The output of the final `pwd` command in this `Dockerfile` would be
`/a/b/c`.

The `WORKDIR` instruction can resolve environment variables previously set using
`ENV`. You can only use environment variables explicitly set in the `Dockerfile`.
For example:

    ENV DIRPATH /path
    WORKDIR $DIRPATH/$DIRNAME
    RUN pwd

The output of the final `pwd` command in this `Dockerfile` would be
`/path/$DIRNAME`

## ARG

    ARG <name>[=<default value>]

The `ARG` instruction defines a variable that users can pass at build-time to
the builder with the `docker build` command using the `--build-arg <varname>=<value>`
flag. If a user specifies a build argument that was not
defined in the Dockerfile, the build outputs a warning.

```
[Warning] One or more build-args [foo] were not consumed.
```

The Dockerfile author can define a single variable by specifying `ARG` once or many
variables by specifying `ARG` more than once. For example, a valid Dockerfile:

```
FROM busybox
ARG user1
ARG buildno
...
```

A Dockerfile author may optionally specify a default value for an `ARG` instruction:

```
FROM busybox
ARG user1=someuser
ARG buildno=1
...
```

If an `ARG` value has a default and if there is no value passed at build-time, the
builder uses the default.

An `ARG` variable definition comes into effect from the line on which it is
defined in the `Dockerfile` not from the argument's use on the command-line or
elsewhere.  For example, consider this Dockerfile:

```
1 FROM busybox
2 USER ${user:-some_user}
3 ARG user
4 USER $user
...
```
A user builds this file by calling:

```
$ docker build --build-arg user=what_user Dockerfile
```

The `USER` at line 2 evaluates to `some_user` as the `user` variable is defined on the
subsequent line 3. The `USER` at line 4 evaluates to `what_user` as `user` is
defined and the `what_user` value was passed on the command line. Prior to its definition by an
`ARG` instruction, any use of a variable results in an empty string.

> **Warning:** It is not recommended to use build-time variables for
>  passing secrets like github keys, user credentials etc. Build-time variable
>  values are visible to any user of the image with the `docker history` command.

You can use an `ARG` or an `ENV` instruction to specify variables that are
available to the `RUN` instruction. Environment variables defined using the
`ENV` instruction always override an `ARG` instruction of the same name. Consider
this Dockerfile with an `ENV` and `ARG` instruction.

```
1 FROM ubuntu
2 ARG CONT_IMG_VER
3 ENV CONT_IMG_VER v1.0.0
4 RUN echo $CONT_IMG_VER
```
Then, assume this image is built with this command:

```
$ docker build --build-arg CONT_IMG_VER=v2.0.1 Dockerfile
```

In this case, the `RUN` instruction uses `v1.0.0` instead of the `ARG` setting
passed by the user:`v2.0.1` This behavior is similar to a shell
script where a locally scoped variable overrides the variables passed as
arguments or inherited from environment, from its point of definition.

Using the example above but a different `ENV` specification you can create more
useful interactions between `ARG` and `ENV` instructions:

```
1 FROM ubuntu
2 ARG CONT_IMG_VER
3 ENV CONT_IMG_VER ${CONT_IMG_VER:-v1.0.0}
4 RUN echo $CONT_IMG_VER
```

Unlike an `ARG` instruction, `ENV` values are always persisted in the built
image. Consider a docker build without the `--build-arg` flag:

```
$ docker build Dockerfile
```

Using this Dockerfile example, `CONT_IMG_VER` is still persisted in the image but
its value would be `v1.0.0` as it is the default set in line 3 by the `ENV` instruction.

The variable expansion technique in this example allows you to pass arguments
from the command line and persist them in the final image by leveraging the
`ENV` instruction. Variable expansion is only supported for [a limited set of
Dockerfile instructions.](#environment-replacement)

Docker has a set of predefined `ARG` variables that you can use without a
corresponding `ARG` instruction in the Dockerfile.

* `HTTP_PROXY`
* `http_proxy`
* `HTTPS_PROXY`
* `https_proxy`
* `FTP_PROXY`
* `ftp_proxy`
* `NO_PROXY`
* `no_proxy`

To use these, simply pass them on the command line using the flag:

```
--build-arg <varname>=<value>
```

### Impact on build caching

`ARG` variables are not persisted into the built image as `ENV` variables are.
However, `ARG` variables do impact the build cache in similar ways. If a
Dockerfile defines an `ARG` variable whose value is different from a previous
build, then a "cache miss" occurs upon its first usage, not its definition. In
particular, all `RUN` instructions following an `ARG` instruction use the `ARG`
variable implicitly (as an environment variable), thus can cause a cache miss.

For example, consider these two Dockerfile:

```
1 FROM ubuntu
2 ARG CONT_IMG_VER
3 RUN echo $CONT_IMG_VER
```

```
1 FROM ubuntu
2 ARG CONT_IMG_VER
3 RUN echo hello
```

If you specify `--build-arg CONT_IMG_VER=<value>` on the command line, in both
cases, the specification on line 2 does not cause a cache miss; line 3 does
cause a cache miss.`ARG CONT_IMG_VER` causes the RUN line to be identified
as the same as running `CONT_IMG_VER=<value>` echo hello, so if the `<value>`
changes, we get a cache miss.

Consider another example under the same command line:

```
1 FROM ubuntu
2 ARG CONT_IMG_VER
3 ENV CONT_IMG_VER $CONT_IMG_VER
4 RUN echo $CONT_IMG_VER
```
In this example, the cache miss occurs on line 3. The miss happens because
the variable's value in the `ENV` references the `ARG` variable and that
variable is changed through the command line. In this example, the `ENV`
command causes the image to include the value.

If an `ENV` instruction overrides an `ARG` instruction of the same name, like
this Dockerfile:

```
1 FROM ubuntu
2 ARG CONT_IMG_VER
3 ENV CONT_IMG_VER hello
4 RUN echo $CONT_IMG_VER
```

Line 3 does not cause a cache miss because the value of `CONT_IMG_VER` is a
constant (`hello`). As a result, the environment variables and values used on
the `RUN` (line 4) doesn't change between builds.

## ONBUILD

    ONBUILD [INSTRUCTION]

The `ONBUILD` instruction adds to the image a *trigger* instruction to
be executed at a later time, when the image is used as the base for
another build. The trigger will be executed in the context of the
downstream build, as if it had been inserted immediately after the
`FROM` instruction in the downstream `Dockerfile`.

Any build instruction can be registered as a trigger.

This is useful if you are building an image which will be used as a base
to build other images, for example an application build environment or a
daemon which may be customized with user-specific configuration.

For example, if your image is a reusable Python application builder, it
will require application source code to be added in a particular
directory, and it might require a build script to be called *after*
that. You can't just call `ADD` and `RUN` now, because you don't yet
have access to the application source code, and it will be different for
each application build. You could simply provide application developers
with a boilerplate `Dockerfile` to copy-paste into their application, but
that is inefficient, error-prone and difficult to update because it
mixes with application-specific code.

The solution is to use `ONBUILD` to register advance instructions to
run later, during the next build stage.

Here's how it works:

1. When it encounters an `ONBUILD` instruction, the builder adds a
   trigger to the metadata of the image being built. The instruction
   does not otherwise affect the current build.
2. At the end of the build, a list of all triggers is stored in the
   image manifest, under the key `OnBuild`. They can be inspected with
   the `docker inspect` command.
3. Later the image may be used as a base for a new build, using the
   `FROM` instruction. As part of processing the `FROM` instruction,
   the downstream builder looks for `ONBUILD` triggers, and executes
   them in the same order they were registered. If any of the triggers
   fail, the `FROM` instruction is aborted which in turn causes the
   build to fail. If all triggers succeed, the `FROM` instruction
   completes and the build continues as usual.
4. Triggers are cleared from the final image after being executed. In
   other words they are not inherited by "grand-children" builds.

For example you might add something like this:

    [...]
    ONBUILD ADD . /app/src
    ONBUILD RUN /usr/local/bin/python-build --dir /app/src
    [...]

> **Warning**: Chaining `ONBUILD` instructions using `ONBUILD ONBUILD` isn't allowed.

> **Warning**: The `ONBUILD` instruction may not trigger `FROM` or `MAINTAINER` instructions.

## STOPSIGNAL

    STOPSIGNAL signal

The `STOPSIGNAL` instruction sets the system call signal that will be sent to the container to exit.
This signal can be a valid unsigned number that matches a position in the kernel's syscall table, for instance 9,
or a signal name in the format SIGNAME, for instance SIGKILL.

## HEALTHCHECK

The `HEALTHCHECK` instruction has two forms:

* `HEALTHCHECK [OPTIONS] CMD command` (check container health by running a command inside the container)
* `HEALTHCHECK NONE` (disable any healthcheck inherited from the base image)

The `HEALTHCHECK` instruction tells Docker how to test a container to check that
it is still working. This can detect cases such as a web server that is stuck in
an infinite loop and unable to handle new connections, even though the server
process is still running.

When a container has a healthcheck specified, it has a _health status_ in
addition to its normal status. This status is initially `starting`. Whenever a
health check passes, it becomes `healthy` (whatever state it was previously in).
After a certain number of consecutive failures, it becomes `unhealthy`.

The options that can appear before `CMD` are:

* `--interval=DURATION` (default: `30s`)
* `--timeout=DURATION` (default: `30s`)
* `--retries=N` (default: `3`)

The health check will first run **interval** seconds after the container is
started, and then again **interval** seconds after each previous check completes.

If a single run of the check takes longer than **timeout** seconds then the check
is considered to have failed.

It takes **retries** consecutive failures of the health check for the container
to be considered `unhealthy`.

There can only be one `HEALTHCHECK` instruction in a Dockerfile. If you list
more than one then only the last `HEALTHCHECK` will take effect.

The command after the `CMD` keyword can be either a shell command (e.g. `HEALTHCHECK
CMD /bin/check-running`) or an _exec_ array (as with other Dockerfile commands;
see e.g. `ENTRYPOINT` for details).

The command's exit status indicates the health status of the container.
The possible values are:

- 0: success - the container is healthy and ready for use
- 1: unhealthy - the container is not working correctly
- 2: reserved - do not use this exit code

For example, to check every five minutes or so that a web-server is able to
serve the site's main page within three seconds:

    HEALTHCHECK --interval=5m --timeout=3s \
      CMD curl -f http://localhost/ || exit 1

To help debug failing probes, any output text (UTF-8 encoded) that the command writes
on stdout or stderr will be stored in the health status and can be queried with
`docker inspect`. Such output should be kept short (only the first 4096 bytes
are stored currently).

When the health status of a container changes, a `health_status` event is
generated with the new status.

The `HEALTHCHECK` feature was added in Docker 1.12.


## SHELL

    SHELL ["executable", "parameters"]

The `SHELL` instruction allows the default shell used for the *shell* form of
commands to be overridden. The default shell on Linux is `["/bin/sh", "-c"]`, and on
Windows is `["cmd", "/S", "/C"]`. The `SHELL` instruction *must* be written in JSON
form in a Dockerfile.

The `SHELL` instruction is particularly useful on Windows where there are
two commonly used and quite different native shells: `cmd` and `powershell`, as
well as alternate shells available including `sh`.

The `SHELL` instruction can appear multiple times. Each `SHELL` instruction overrides
all previous `SHELL` instructions, and affects all subsequent instructions. For example:

    FROM microsoft/windowsservercore

    # Executed as cmd /S /C echo default
    RUN echo default

    # Executed as cmd /S /C powershell -command Write-Host default
    RUN powershell -command Write-Host default

    # Executed as powershell -command Write-Host hello
    SHELL ["powershell", "-command"]
    RUN Write-Host hello

    # Executed as cmd /S /C echo hello
    SHELL ["cmd", "/S"", "/C"]
    RUN echo hello

The following instructions can be affected by the `SHELL` instruction when the
*shell* form of them is used in a Dockerfile: `RUN`, `CMD` and `ENTRYPOINT`.

The following example is a common pattern found on Windows which can be
streamlined by using the `SHELL` instruction:

    ...
    RUN powershell -command Execute-MyCmdlet -param1 "c:\foo.txt"
    ...

The command invoked by docker will be:

    cmd /S /C powershell -command Execute-MyCmdlet -param1 "c:\foo.txt"

This is inefficient for two reasons. First, there is an un-necessary cmd.exe command
processor (aka shell) being invoked. Second, each `RUN` instruction in the *shell*
form requires an extra `powershell -command` prefixing the command.

To make this more efficient, one of two mechanisms can be employed. One is to
use the JSON form of the RUN command such as:

    ...
    RUN ["powershell", "-command", "Execute-MyCmdlet", "-param1 \"c:\\foo.txt\""]
    ...

While the JSON form is unambiguous and does not use the un-necessary cmd.exe,
it does require more verbosity through double-quoting and escaping. The alternate
mechanism is to use the `SHELL` instruction and the *shell* form,
making a more natural syntax for Windows users, especially when combined with
the `escape` parser directive:

    # escape=`

    FROM microsoft/nanoserver
    SHELL ["powershell","-command"]
    RUN New-Item -ItemType Directory C:\Example
    ADD Execute-MyCmdlet.ps1 c:\example\
    RUN c:\example\Execute-MyCmdlet -sample 'hello world'

Resulting in:

    PS E:\docker\build\shell> docker build -t shell .
    Sending build context to Docker daemon 4.096 kB
    Step 1/5 : FROM microsoft/nanoserver
     ---> 22738ff49c6d
    Step 2/5 : SHELL powershell -command
     ---> Running in 6fcdb6855ae2
     ---> 6331462d4300
    Removing intermediate container 6fcdb6855ae2
    Step 3/5 : RUN New-Item -ItemType Directory C:\Example
     ---> Running in d0eef8386e97
    
    
        Directory: C:\
    
    
    Mode                LastWriteTime         Length Name
    ----                -------------         ------ ----
    d-----       10/28/2016  11:26 AM                Example
    
    
     ---> 3f2fbf1395d9
    Removing intermediate container d0eef8386e97
    Step 4/5 : ADD Execute-MyCmdlet.ps1 c:\example\
     ---> a955b2621c31
    Removing intermediate container b825593d39fc
    Step 5/5 : RUN c:\example\Execute-MyCmdlet 'hello world'
     ---> Running in be6d8e63fe75
    hello world
     ---> 8e559e9bf424
    Removing intermediate container be6d8e63fe75
    Successfully built 8e559e9bf424
    PS E:\docker\build\shell>

The `SHELL` instruction could also be used to modify the way in which
a shell operates. For example, using `SHELL cmd /S /C /V:ON|OFF` on Windows, delayed
environment variable expansion semantics could be modified.

The `SHELL` instruction can also be used on Linux should an alternate shell be
required such as `zsh`, `csh`, `tcsh` and others.

The `SHELL` feature was added in Docker 1.12.

## Dockerfile examples

Below you can see some examples of Dockerfile syntax. If you're interested in
something more realistic, take a look at the list of [Dockerization examples](https://docs.docker.com/engine/examples/).

```
# Nginx
#
# VERSION               0.0.1

FROM      ubuntu
LABEL Description="This image is used to start the foobar executable" Vendor="ACME Products" Version="1.0"
RUN apt-get update && apt-get install -y inotify-tools nginx apache2 openssh-server
```

```
# Firefox over VNC
#
# VERSION               0.3

FROM ubuntu

# Install vnc, xvfb in order to create a 'fake' display and firefox
RUN apt-get update && apt-get install -y x11vnc xvfb firefox
RUN mkdir ~/.vnc
# Setup a password
RUN x11vnc -storepasswd 1234 ~/.vnc/passwd
# Autostart firefox (might not be the best way, but it does the trick)
RUN bash -c 'echo "firefox" >> /.bashrc'

EXPOSE 5900
CMD    ["x11vnc", "-forever", "-usepw", "-create"]
```

```
# Multiple images example
#
# VERSION               0.1

FROM ubuntu
RUN echo foo > bar
# Will output something like ===> 907ad6c2736f

FROM ubuntu
RUN echo moo > oink
# Will output something like ===> 695d7793cbe4

# You᾿ll now have two images, 907ad6c2736f with /bar, and 695d7793cbe4 with
# /oink.
```
