docker/man/Dockerfile.5.md

% DOCKERFILE(5) Docker User Manuals
% Zac Dover
% May 2014
# NAME

Dockerfile - automate the steps of creating a Docker image

# INTRODUCTION

The **Dockerfile** is a configuration file that automates the steps of creating
a Docker image. It is similar to a Makefile. Docker reads instructions from the
**Dockerfile** to automate the steps otherwise performed manually to create an
image. To build an image, create a file called **Dockerfile**.

The **Dockerfile** describes the steps taken to assemble the image. When the
**Dockerfile** has been created, call the `docker build` command, using the
path of directory that contains **Dockerfile** as the argument.

# SYNOPSIS

INSTRUCTION arguments

For example:

  FROM image

# DESCRIPTION

A Dockerfile is a file that automates the steps of creating a Docker image. 
A Dockerfile is similar to a Makefile.

# USAGE

  docker build .

  -- Runs the steps and commits them, building a final image.
  The path to the source repository defines where to find the context of the
  build. The build is run by the Docker daemon, not the CLI. The whole
  context must be transferred to the daemon. The Docker CLI reports
  `"Sending build context to Docker daemon"` when the context is sent to the
  daemon.

  ```
  docker build -t repository/tag .
  ```

  -- specifies a repository and tag at which to save the new image if the build
  succeeds. The Docker daemon runs the steps one-by-one, committing the result
  to a new image if necessary, before finally outputting the ID of the new
  image. The Docker daemon automatically cleans up the context it is given.

  Docker re-uses intermediate images whenever possible. This significantly
  accelerates the *docker build* process.

# FORMAT

  `FROM image`

  `FROM image:tag`

  `FROM image@digest`

  -- The **FROM** instruction sets the base image for subsequent instructions. A
  valid Dockerfile must have **FROM** as its first instruction. The image can be any
  valid image. It is easy to start by pulling an image from the public
  repositories.

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

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

  -- If no tag is given to the **FROM** instruction, Docker applies the 
  `latest` tag. If the used tag does not exist, an error is returned.

  -- If no digest is given to the **FROM** instruction, Docker applies the 
  `latest` tag. If the used tag does not exist, an error is returned.

**MAINTAINER**
  -- **MAINTAINER** sets the Author field for the generated images.
  Useful for providing users with an email or url for support.

**RUN**
  -- **RUN** has two forms:

  ```
  # the command is run in a shell - /bin/sh -c
  RUN <command>

  # Executable form
  RUN ["executable", "param1", "param2"]
  ```


  -- The **RUN** instruction executes any commands in a new layer on top of the current
  image and commits the results. The committed image is used for the next step in
  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 the history of an image. This is similar to source control.  The
  exec form makes it possible to avoid shell string munging. The exec form makes
  it possible to **RUN** commands using a base image that does not contain `/bin/sh`.

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

**CMD**
  -- **CMD** has three forms:

  ```
  # Executable form
  CMD ["executable", "param1", "param2"]`

  # Provide default arguments to ENTRYPOINT
  CMD ["param1", "param2"]`

  # the command is run in a shell - /bin/sh -c
  CMD command param1 param2
  ```

  -- There should be only one **CMD** in a Dockerfile. If more than one **CMD** is listed, only
  the last **CMD** takes effect.
  The main purpose of a **CMD** is to provide defaults for an executing container.
  These defaults may include an executable, or they can omit the executable. If
  they omit the executable, an **ENTRYPOINT** must be specified.
  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**, the `<command>` executes in `/bin/sh -c`:

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

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

  -- If you run **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 form of **CMD**. All additional parameters must be individually expressed
  as strings in the array:

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

  -- To make the container run the same executable every time, use **ENTRYPOINT** in
  combination with **CMD**. 
  If the user specifies arguments to `docker run`, the specified commands
  override the default in **CMD**.
  Do not confuse **RUN** with **CMD**. **RUN** runs a command and commits the result.
  **CMD** executes nothing at build time, but specifies the intended command for
  the image.

**LABEL**
  -- `LABEL <key>=<value> [<key>=<value> ...]`or 
  ```
  LABEL <key>[ <value>]
  LABEL <key>[ <value>]
  ...
  ```
  The **LABEL** instruction adds metadata to an image. A **LABEL** is a
  key-value pair. To specify a **LABEL** without a value, simply use an empty
  string. To include spaces within a **LABEL** value, use quotes and
  backslashes as you would in command-line parsing.

  ```
  LABEL com.example.vendor="ACME Incorporated"
  LABEL com.example.vendor "ACME Incorporated"
  LABEL com.example.vendor.is-beta ""
  LABEL com.example.vendor.is-beta=
  LABEL com.example.vendor.is-beta=""
  ```

  An image can have more than one label. To specify multiple labels, separate
  each key-value pair by a space. 
  
  Labels are additive including `LABEL`s in `FROM` images. As the system
  encounters and then applies a new label, new `key`s override any previous
  labels with identical keys.

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

**EXPOSE**
  -- `EXPOSE <port> [<port>...]`
  The **EXPOSE** instruction informs Docker that the container listens on the
  specified network ports at runtime. Docker uses this information to
  interconnect containers using links and to set up port redirection on the host
  system.

**ENV**
  -- `ENV <key> <value>`
  The **ENV** instruction sets the environment variable <key> to
  the value `<value>`. This value is passed to all future 
  **RUN**, **ENTRYPOINT**, and **CMD** instructions. This is
  functionally equivalent to prefixing the command with `<key>=<value>`.  The
  environment variables that are set with **ENV** persist when a container is run
  from the resulting image. Use `docker inspect` to inspect these values, and
  change them using `docker run --env <key>=<value>`.

  Note that setting "`ENV DEBIAN_FRONTEND noninteractive`" may cause
  unintended consequences, because it will persist when the container is run
  interactively, as with the following command: `docker run -t -i image bash`

**ADD**
  -- **ADD** has two forms:

  ```
  ADD <src> <dest>

  # Required for paths with whitespace
  ADD ["<src>",... "<dest>"]
  ```

  The **ADD** instruction copies new files, directories
  or remote file URLs to the filesystem of the container at path `<dest>`.
  Multiple `<src>` resources 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). The `<dest>` is the absolute path, or path relative
  to **WORKDIR**, into which the source is copied inside the target container.
  If the `<src>` argument is a local file in a recognized compression format
  (tar, gzip, bzip2, etc) then it is unpacked at the specified `<dest>` in the
  container's filesystem.  Note that only local compressed files will be unpacked,
  i.e., the URL download and archive unpacking features cannot be used together.
  All new directories are created with mode 0755 and with the uid and gid of **0**.

**COPY**
  -- **COPY** has two forms:

  ```
  COPY <src> <dest>

  # Required for paths with whitespace
  COPY ["<src>",... "<dest>"]
  ```

  The **COPY** instruction copies new files from `<src>` and
  adds them to the filesystem of the container at path <dest>. The `<src>` must be
  the path to a file or directory relative to the source directory that is
  being built (the context of the build) or a remote file URL. The `<dest>` is an
  absolute path, or a path relative to **WORKDIR**, into which the source will
  be copied inside the target container. If you **COPY** an archive file it will
  land in the container exactly as it appears in the build context without any 
  attempt to unpack it.  All new files and directories are created with mode **0755**
  and with the uid and gid of **0**.

**ENTRYPOINT**
  -- **ENTRYPOINT** has two forms:

  ```
  # executable form
  ENTRYPOINT ["executable", "param1", "param2"]`

  # run command in a shell - /bin/sh -c
  ENTRYPOINT command param1 param2
  ```

  -- An **ENTRYPOINT** helps you configure a
  container that can be run as an executable. When you specify an **ENTRYPOINT**,
  the whole container runs as if it was only that executable.  The **ENTRYPOINT**
  instruction adds an entry command that is not overwritten when arguments are
  passed to docker run. This is different from the behavior of **CMD**. This allows
  arguments to be passed to the entrypoint, for instance `docker run <image> -d`
  passes the -d argument to the **ENTRYPOINT**.  Specify parameters either in the
  **ENTRYPOINT** JSON array (as in the preferred exec form above), or by using a **CMD**
  statement.  Parameters in the **ENTRYPOINT** are not overwritten by the docker run
  arguments.  Parameters specified via **CMD** are overwritten by docker run
  arguments.  Specify a plain string for the **ENTRYPOINT**, and it will execute in
  `/bin/sh -c`, like a **CMD** instruction:

  ```
  FROM ubuntu
  ENTRYPOINT wc -l -
  ```

  This means that the Dockerfile's image always takes stdin as input (that's
  what "-" means), and prints the number of lines (that's what "-l" means). To
  make this optional but default, use a **CMD**:

  ```
  FROM ubuntu
  CMD ["-l", "-"]
  ENTRYPOINT ["/usr/bin/wc"]
  ```

**VOLUME**
  -- `VOLUME ["/data"]`
  The **VOLUME** instruction creates a mount point with the specified name and marks
  it as holding externally-mounted volumes from the native host or from other
  containers.

**USER**
  -- `USER daemon`
  Sets the username or UID used for running subsequent commands.

  The **USER** instruction can optionally be used to set the group or GID. The
  followings examples are all valid:
  USER [user | user:group | uid | uid:gid | user:gid | uid:group ]

  Until the **USER** instruction is set, instructions will be run as root. The USER
  instruction can be used any number of times in a Dockerfile, and will only affect
  subsequent commands.

**WORKDIR**
  -- `WORKDIR /path/to/workdir`
  The **WORKDIR** instruction sets the working directory for the **RUN**, **CMD**,
  **ENTRYPOINT**, **COPY** and **ADD** Dockerfile commands that follow it. It can
  be used multiple times in a single Dockerfile. Relative paths are defined
  relative to the path of the previous **WORKDIR** instruction. For example:

  ```
  WORKDIR /a
  WORKDIR b
  WORKDIR c
  RUN pwd
  ```

  In the above example, the output of the **pwd** command is **a/b/c**.

**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 an error.

  ```
  One or more build-args were not consumed, failing build.
  ```

  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.

  > **Note:** It is not recommended to use build-time variables for
  >  passing secrets like github keys, user credentials etc.

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

**ONBUILD**
  -- `ONBUILD [INSTRUCTION]`
  The **ONBUILD** instruction adds a trigger instruction to an image. The
  trigger is executed at a later time, when the image is used as the base for
  another build. Docker executes the trigger in the context of the downstream
  build, as if the trigger existed immediately after the **FROM** instruction in
  the downstream Dockerfile.

  You can register any build instruction as a trigger. A trigger is useful if
  you are defining an image to use as a base for building other images. For
  example, if you are defining an application build environment or a daemon that
  is customized with a user-specific configuration.  
  
  Consider an image intended as a reusable python application builder. It must
  add application source code to a particular directory, and might need a build
  script 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 is different
  for each application build.

  -- Providing application developers with a boilerplate Dockerfile to copy-paste
  into their application is inefficient, error-prone, and
  difficult to update because it mixes with application-specific code.
  The solution is to use **ONBUILD** to register instructions in advance, to
  run later, during the next build stage.

# HISTORY
*May 2014, Compiled by Zac Dover (zdover at redhat dot com) based on docker.com Dockerfile documentation.
*Feb 2015, updated by Brian Goff (cpuguy83@gmail.com) for readability
*Sept 2015, updated by Sally O'Malley (somalley@redhat.com)