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.
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.
Refer to the “build images with BuildKit” section to learn about secure ways to use secrets when building images.
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>.
If an environment variable is only needed during build, and not in the final image, consider setting a value for a single command instead. Or using ARG, which is not persisted in the final image: ARG DEBIAN_FRONTEND=noninteractive
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.
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
ADD [--chown=<user>:<group>] <src>... <dest>
ADD [--chown=<user>:<group>] ["<src>",... "<dest>"]
The latter 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>.
The <dest> is an absolute path, or a path relative to WORKDIR, into which the source will be copied inside the destination container.
ADD node_modules /myCompany/node_modules // What the fuck, we don't need node_modules in production
ADD dist /myCompany/dist // Not a good way too, be better to use volume for this
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 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 mountpoint at /myvol and copy the greeting file into the newly created volume.
The host directory is declared at container run-time: The host directory (the mountpoint) is, by its nature, host-dependent. This is to preserve image portability, since a given host directory can’t be guaranteed to be available on all hosts. For this reason, you can’t mount a host directory from within the Dockerfile. The VOLUME instruction does not support specifying a host-dir parameter. You must specify the mountpoint when you create or run the container.
Start a container with a volume
If you start a container with a volume that does not yet exist, Docker creates the volume for you
docker run -d --name devtest --mount type=volume,source=my-vol,destination=/myvol nginx:latest
docker run -d --name devtest --volume my-vol:/myvol nginx:latest
docker inspect devtest
"Mounts": [
{
"Type": "volume",
"Name": "my-vol",
"Source": "/var/lib/docker/volumes/my-vol/_data",
"Destination": "/myvol",
}
],
-v or --volume, Consists of three fields, separated by colon characters (:). The fields must be in the correct order:
In the case of named volumes, the first field is the name of the volume, and is unique on a given host machine. For anonymous volumes, the first field is omitted.
In the case of bind mounts, the first field is the path to the file or directory on the host machine. You can’t use Docker CLI commands to directly manage bind mounts.
you can create and manage volumes outside the scope of any container:
docker volume create my-vol
docker volume inspect my-vol
"Mountpoint": "/var/lib/docker/volumes/my-vol/_data",
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)
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.
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.
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 used in the shell or exec formats, the CMD instruction sets the command to be executed when running the image.
If the user specifies arguments to docker run then they will override the default specified in CMD.
Do not confuse RUN with CMD. RUN actually runs a command and commits the result; CMD does not execute anything at build time.
The exec form: ENTRYPOINT ["executable", "param1", "param2"]
The shell form: ENTRYPOINT command param1 param2
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. You can override the ENTRYPOINT instruction using the docker run --entrypoint flag.
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.
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. CMD will be overridden when running the container with alternative arguments.
If CMD is defined from the base image, setting ENTRYPOINT will reset CMD to an empty value. In this scenario, CMD must be defined in the current image to have a value.
EXPOSE
EXPOSE <port> [<port>/<protocol>...]
The EXPOSE instruction informs Docker that the container listens on the specified network ports at runtime. You can specify whether the port listens on TCP or UDP, and the default is TCP if the protocol is not specified.
The EXPOSE instruction does not actually publish the port. It functions as a type of documentation between the person who builds the image and the person who runs the container, about which ports are intended to be published. To actually publish the port when running the container, use the -p flag on docker run to publish and map one or more ports, or the -P flag to publish all exposed ports and map them to high-order ports.
EXPOSE 80/udp
To expose on both TCP and UDP, include two lines:
EXPOSE 80/tcp
EXPOSE 80/udp
In this case, if you use -P with docker run, the port will be exposed once for TCP and once for UDP. Remember that -P uses an ephemeral high-ordered host port on the host, so the port will not be the same for TCP and UDP.
Regardless of the EXPOSE settings, you can override them at runtime by using the -p flag. For example
docker run -p 80:80/tcp -p 80:80/udp …