ARGs are not handled in FROM instruction about dockerfile-parse

[twaugh]

I think there are two causes:

• we only handle ENV variables, not ARG variables
• we only perform variable expansion for ENV and LABEL instructions

[twaugh]

Would you be able to have a go at contributing support for this?

[nishakm]

👋 I don't think this is solvable before build time. Per the Dockerfile reference ARG is evaluated at build time based on what you pass to docker build --build-args. In this case ${PYTHON_VERSION} is only known at build time. Maybe if you gave the tool both the Dockerfile and the build command, it can do the appropriate substitution.

[MartinBasti]

@nishakm
I'd substitute only defined ARG in dockerfile, eventually we can add extra param build_args to related methods.

[nishakm]

@MartinBasti sounds good. Is anyone working on this? I can submit a PR if that's OK.

[MartinBasti]

Feel free to submit PR :)