subtests.docker_cli: Add new test run_volumes_selinux

This test uses `docker run --volume ::Z/z` and checks it sets the
selinux context properly.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Yep, I understand why, it's just an area to be mindful of, some environments for testing are less-flexible than others.  Though I did check just now, at at least with the recent Atomic images, we do include python-selinux.  So what you have here is okay for now.