Add own container with systemd, so we can (in a follow-up commit) run
the Osmocom systemd services in this test job. Rewrite the
"interactive shell" logic to support the new systemd docker container,
and enable it with an INTERACTIVE environment variable instead of
hardcoding 'interactive="true"' in the script. While at it, move the
Repository.key install to the Dockerfile so it works more like the other
docker containers we have.
Related: OS#3369
Change-Id: I0348f65a2ac184ba6b001e5130dfc8124e657367
