Add a container build test
    
    This test, much like _live_build or _installer_build, builds a
    container image in a way intended to be as similar as possible
    to how official compose images are built. The purpose of the test
    is to make sure updates do not break official container image
    builds.
    
    At the end of the test, we also check that the built container
    is functional (at least, that we can run a 'hello world' command
    in it). This can't really be rolled into podman.pm because that
    test is more about testing podman itself, and it's just a one-
    liner here anyway. We also run the 'if any packages from the
    update are installed, are they the versions from the update?'
    check inside the container, which required giving that check the
    ability to 'wrap' the rpm commands to run inside a container.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>