Investigate and document why FORCE_IMAGE_BUILDS variable is set on project mirrors

FORCE_IMAGE_BUILDS variable is set on both Canonical and Build mirror. This causes images to be rebuilt from scratch even if nothing essentially changed - thus nullifying the reasoning why we have images tagged and pushed with CONTAINER_VERSION.

We should reconfirm that we actually want this, and document it if that is the case. Personally, I feel only nightlies need to attempt to build everything from scratch - to ensure there are no hidden failures that will bite us during release. Everything else should be able to skip image building if the images already exist.