registry: Change the way in which images are tagged

Previously we tried using the git commit ref as a UID for the images. This does not work though cause multiple jobs that rebuild the image can be triggered and override the same image tag. Instead use the CI_JOB_ID to provide better semantics wrt naming conflicts.

Additionally add the date as part of the tag to better indicate the age of the image. Gitlab WEBGUI doens't indicate the age in a good way nor makes it easy to short the images.