migrate osmo-gsm-tester docker images to registry.osmocom.org
As of #4839, we now have registry.osmocom.org. Hence, it makes sense to stop using sysmocom infrastructure (registry.sysmocom.de) for osmo-gsm-testerm image storage and migrate over.
The same "docker login" credentials should work on regsitry.osmocom.org.
Please make sure to notify laforge after the migration, so the old images can be deleted from registry.sysmocom.de