Save build images into Harbor tagged with the build id.

[tobybellwood]

Currently, Lagoon only generates a latest tag for images pushed to the Harbor registry.

In order to better connect builds with their resultant images, a tag associating the build with its specific image digest should also be pushed to Harbor. This won't take up more space (because of layers), but will aid in case the image ever needs retrieving from Harbor for any given build, or if the docker_host needs checking for a recently built image (e.g #195)