Encode build information into docker images as labels

[glance-]

Our current docker image builder code uses tags to encode which build number, git hash and git branch the image was built from.

The problem with using tags is that when only one tag is pulled, the rest of the information is lost.

I propose that we switch over to using build time labels on the docker image to encode that information instead. Those won't fall of the image as tags might do, and will follow the image, no matter how the image moves around in different systems.

I've started working on adding support for labels in:
jenkinsci/docker-plugin#769

If/when that code is done/merged, it will probably be quite easy to start using it in our build jobs.