% "CONTAINERFILE" "5" "Aug 2021" "" "Container User Manuals"

# NAME

Containerfile(Dockerfile) - automate the steps of creating a container image

# INTRODUCTION

The **Containerfile** is a configuration file that automates the steps of creating a container image. It is similar to a Makefile. Container engines (Podman, Buildah, Docker) read instructions from the **Containerfile** to automate the steps otherwise performed manually to create an image. To build an image, create a file called **Containerfile**.

The **Containerfile** describes the steps taken to assemble the image. When the

**Containerfile** has been created, call the `buildah bud`, `podman build`, `docker build` command,

using the path of context directory that contains **Containerfile** as the argument. Podman and Buildah default to **Containerfile** and will fall back to **Dockerfile**. Docker only will search for **Dockerfile** in the context directory.

**Dockerfile** is an alternate name for the same object.  **Containerfile** and **Dockerfile** support the same syntax.

# SYNOPSIS

INSTRUCTION arguments

For example:

  FROM image

# DESCRIPTION

A Containerfile is a file that automates the steps of creating a container image.

A Containerfile is similar to a Makefile.

# USAGE

  ```

  buildah bud .

  podman 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.

  ```

  buildah bud -t repository/tag .

  podman build -t repository/tag .

  ```

  -- specifies a repository and tag at which to save the new image if the build

  succeeds. The container engine runs the steps one-by-one, committing the result

  to a new image if necessary, before finally outputting the ID of the new

  image.

  Container engines re-use intermediate images whenever possible. This significantly

  accelerates the *build* process.

# FORMAT

  `FROM image`

  `FROM image:tag`

  `FROM image@digest`

  -- The **FROM** instruction sets the base image for subsequent instructions. A

  valid Containerfile 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 Containerfile.

  -- **FROM** may appear multiple times within a single Containerfile 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, container engines apply the

  `latest` tag. If the used tag does not exist, an error is returned.

  -- If no digest is given to the **FROM** instruction, container engines apply 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"]

  ```

**RUN Secrets*

The RUN command has a feature to allow the passing of secret information into the image build. These secrets files can be used during the RUN command but are not committed to the final image. The `RUN` command supports the `--mount` option to identify the secret file. A secret file from the host is mounted into the container while the image is being built.

Container engines pass secret the secret file into the build using the `--secret` flag.

**RUN --mount* options:

- `id` is the identifier to for the secret passed into the `buildah bud --secret` or `podman build --secret`. This identifier is associated with the RUN --mount identifier to use in the Containerfile.

- `dst`|`target`|`destination` rename the secret file to a specific file in the Containerfile RUN command to use.

- `type=secret` tells the --mount command that it is mounting in a secret file

# shows secret from default secret location:

RUN --mount=type=secret,id=mysecret cat /run/secrets/mysecret

# shows secret from custom secret location:

RUN --mount=type=secret,id=mysecret,dst=/foobar cat /foobar

The secret needs to be passed to the build using the --secret flag. The final image built does not container the secret file:

```

 buildah bud --no-cache --secret id=mysecret,src=mysecret.txt .

```

  -- 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

  Containerfile.

  -- Layering **RUN** instructions and generating commits conforms to the core

  concepts of container engines 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 Containerfile. 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 `podman run` or `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 `buildah inspect` command.

**EXPOSE**

  -- `EXPOSE <port> [<port>...]`

  The **EXPOSE** instruction informs the container engine that the container listens on the

  specified network ports at runtime. The container engine 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 `podman inspect` to inspect these values, and

  change them using `podman 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: `podman 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 `podman run`. This is different from the behavior of **CMD**. This allows

  arguments to be passed to the entrypoint, for instance `podman 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 `podman run` arguments.  Parameters specified via **CMD** are overwritten by `podman 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 Containerfile'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

  following 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 Containerfile, 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** Containerfile commands that follow it. It can

  be used multiple times in a single Containerfile. 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 `podman build` command using the `--build-arg

  <varname>=<value>` flag. If a user specifies a build argument that was not

  defined in the Containerfile, the build outputs a warning.

  ```

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

  ```

  The Containerfile author can define a single variable by specifying `ARG` once or many

  variables by specifying `ARG` more than once. For example, a valid Containerfile:

  ```

  FROM busybox

  ARG user1

  ARG buildno

  ...

  ```

  A Containerfile 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 `Containerfile` not from the argument's use on the command-line or

  elsewhere.  For example, consider this Containerfile:

  ```

  1 FROM busybox

  2 USER ${user:-some_user}

  3 ARG user

  4 USER $user

  ...

  ```

  A user builds this file by calling:

  ```

  $ podman build --build-arg user=what_user Containerfile

  ```

  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 `podman 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 Containerfile 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:

  ```

  $ podman build --build-arg CONT_IMG_VER=v2.0.1 Containerfile

  ```

  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 `podman build` without the --build-arg flag:

  ```

  $ podman build Containerfile

  ```

  Using this Containerfile 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

  Containerfile instructions.](#environment-replacement)

  Container engines have a set of predefined `ARG` variables that you can use without a

  corresponding `ARG` instruction in the Containerfile.

  * `HTTP_PROXY`

  * `http_proxy`

  * `HTTPS_PROXY`

  * `https_proxy`

  * `FTP_PROXY`

  * `ftp_proxy`

  * `NO_PROXY`

  * `no_proxy`

  * `ALL_PROXY`

  * `all_proxy`

  To use these, pass them on the command line using `--build-arg` flag, for

  example:

  ```

  $ podman build --build-arg HTTPS_PROXY=https://my-proxy.example.com .

  ```

**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. Container engines execute the trigger in the context of the downstream

  build, as if the trigger existed immediately after the **FROM** instruction in

  the downstream Containerfile.

  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 Containerfile 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.

## SEE ALSO

buildah(1), podman(1), docker(1)

# 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)

*Oct 2016, updated by Addam Hardy (addam.hardy@gmail.com)

*Aug 2021, converted Dockerfile man page to Containerfile by Dan Walsh (dwalsh@redhat.com)