% "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 either **ARG** or *FROM** as its first instruction.
If **FROM** is not the first instruction in the file, it may only be preceded by
one or more ARG instructions, which declare arguments that are used in the next FROM line in the Containerfile.
The image can be any valid image. It is easy to start by pulling an image from the public
repositories.
-- **FROM** must appear at least once in the Containerfile.
-- **FROM** The first **FROM** command must come before all other instructions in
the Containerfile except **ARG**
-- **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.
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.
-`id` is the identifier 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:
-- 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
