Add basic rendering tests with tox

[Jamim]

Hello,

I believe it would be nice to have at least some basic rendering tests for pambase. So here it is :-)
There is a bit related bug: https://bugs.gentoo.org/916869

I realize this is a mirror repository, but I'm not sure if there's a better way to propose these changes.

Best regards!

[thesamesam]

Big thanks for doing this. I've been nervous about touching pambase because it's so hard to verify the stability of results, which is of course critical because PAM's rules are line-order based.

Would you consider adding a basic github action rule to run tox?

[Jamim]

I've committed changes those enable tests on the GitHub side.
The implementation relies on Official Gentoo Docker images, since I believe it's a better way to test Gentoo-specific packages.

A workflow outcome sample might be reviewed here:
https://github.com/Jamim/pambase/actions/runs/6800065146/job/18487784920

Useful links:

• https://www.gentoo.org/news/2020/07/04/official-docker.html
• https://github.com/gentoo/gentoo-docker-images
• https://github.com/docker/build-push-action
• https://docs.docker.com/build/ci/github-actions/cache/

[Jamim]

By the way, I've found there supposed to be gentoo/python images.
They would be quite useful for this PR, but I can't find them on Docker Hub.

cc @mgorny

[thesamesam]

Thank you again!