Update containers and include CI test for container builds

[gunnarx]

Significant update of the docker/container files.

Primary reason for the update:

1. Complete and bugfix the configuration of Dockerfiles and Makefile for the containers.
2. Introduce a Github-Actions CI workflow to ensure that building containers keeps working, and also to run pytest for IFEX in the container environment(s).

(Some) detailed changes:

• Parameterize docker/podman/whatever frontend is desired.
• Simplify the specification of preferred python version. Having a .dotfile to specify the version turned out to have different behavior locally vs GitHub actions, so it's simply coded in the Dockerfiles now.

Why? -> The container setup enables:

• (now) A "guaranteed to work" method to run IFEX tools, if other installation methods fail.
• (now) Isolated execution environment, so when updating the Python version, we can know we are testing with (only) that version on the system and not some other system-installed python files.
• (now) Try out different ways of installing and setting up the python environment, for example pyenv (installing python, mostly) and virtualenv (installing dependencies in local env, mostly).
• (now) Better understand if there are particulars to know for a certain distro (currently only Ubuntu and Alpine container versions)
• (future) Prepare for possibility to test over many python versions, to catch regressions, unsupported language features, or other issues. Although, that could also be done completely within GitHub actions workflows.

NOTES:

• The Ubuntu container version uses pyenv which installs/builds Python from source code.
• The Alpine version uses a pre-baked binary release and a virtual environment.

It's good because it creates diversity in testing, but the original reason was to work around bugs with pyenv / compiling python from source on the alpine variant.

[gunnarx]

NOTE: The container builds/tests are a bit heavier on network/execution time, so they run on pull requests only, compared to the traditional unit test and docs generation that runs also on pushes.