Do not build debian-stretch-titan, when registry.osmocom.org is enabled. The image that would be built at this point is not useful, since other images like ttcn3-msc-test will use the debian-stretch-titan image from the registry instead of the one that was just built locally.
Pull debian-stretch-titan from registry.osmocom.org if the repository is enabled. Otherwise the image gets only pulled the first time, and does not get updated once it becomes outdated.
