Docker Command¶

ARG vs ENV¶

https://vsupalov.com/docker-arg-env-variable-guide/
https://docs.docker.com/engine/reference/builder/
https://stackoverflow.com/questions/63278658/build-and-push-a-docker-image-with-build-arguments-from-devops-to-acr

Feature	`ARG`	`ENV`
Scope	Build-time only	Runtime and build-time
Default Value	Can be set with `ARG <name>=<val>`	Can be set with `ENV <name>=<val>`
Overridable	Overridden with `--build-arg`	Overridden with `docker run -e`
Persisted in Image	❌ Not saved in final image	✅ Saved in image and available at runtime
Use in RUN	✅ Yes	✅ Yes
Use in CMD/ENTRYPOINT	❌ Not available	✅ Available

arg¶

Pass argument

arguments: '--build-arg ubuntu_version=20.04 --build-arg image_version=1.0.0'

ARG VERS=22.04
FROM ubuntu:$VERS
ARG VERS
RUN echo version is $VERS

Second VERS will get default value from the first global one
The ARG statement after a FROM is required to import the value into the build target

ADD vs COPY¶

https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#:~:text=ADD%20or%20COPY&text=COPY%20only%20supports%20the%20basic,rootfs.tar.xz%20%2F%20.

ADD: has some features (like local-only tar extraction and remote URL support)
COPY: only supports the basic copying of local files into the container, preferred as it’s more transparent

RUN¶

Mainly used to build images and install applications and packages, creating a new layer.

WORKDIR¶

Define the working directory of a Docker container at any given time
Any RUN, CMD, ADD, COPY, or ENTRYPOINT command will be executed in the specified working directory
If reused, the path of the new working directory must be given relative to the current working directory
```
ARG USER="user-name"
ARG HOME="/home/${USER}"
WORKDIR $HOME
```

CMD vs ENTRYPOINT¶

CMD: Default parameters that can be overridden from the Docker CLI when a container is running.
ENTRYPOINT: Default parameters that cannot be overridden when the Docker Container runs with CLI parameters.
CMD provides default arguments to the ENTRYPOINT.

Feature	`CMD`	`ENTRYPOINT`
Purpose	Default command + args to run if no command is given during `docker run`	Command that always runs when container starts
Can be overridden	Yes — by providing command/args in `docker run`	Yes — but you must explicitly override with `--entrypoint`
Syntax forms	Shell form (string), Exec form (list)	Exec form (list) recommended
Typical usage	Default command with default args	Main executable, often for making the container behave like an executable

Example #1:

ENTRYPOINT ["python", "app.py"]
CMD ["--help"]

If you run docker run my-image, it runs: python app.py --help
If you run docker run my-image --version, it runs: python app.py --version

Example #2

COPY docker/entrypoint.sh /
ENTRYPOINT ["/entrypoint.sh"]
CMD ["/bin/bash"]