Run test_base_size only on podman

[grisu48]

Due to inconsistent space reporting between docker and podman we limit the image size test to podman only.

• Context: https://github.com/containers/podman/issues/13777#issuecomment-1090133955
• Related failures: https://openqa.suse.de/tests/14103942#step/_root_BCI-tests_base_/2

[grisu48]

Amended and rebased.