This is initialized to docker.io, keeping the default behaviour
if not specified. However, it allows us to specify a private
registry later on.
Related: OS#4839
Change-Id: I32d4ee6256033c809108c1b86cb6b6c58d880f49
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