Add supoort for enabling GPU access

[mokeyish]

closes #919

[mokeyish]

@p12tic I looked at the existing unit tests and it was a bit difficult to understand. It seems to be output after execution. But Github Actions does not have a GPU and cannot be tested.

[p12tic]

@mokeyish Tests in pytests/test_container_to_args.py don't actually run any containers.

The test structure is as follows:

• create compose definition in parsed form (i.e. python dicts, lists, etc. are used)
• run it through container_to_args function
• the result is arguments which would be otherwise sent to podman. The test simply compares them to what's expected and does not run podman.

With such tests any feature of podman-compose can be tested without any requirements from hardware.

[p12tic]

So in GPU tests I would expect tests that check whether correct GPU-related options are added to podman_args.