gitlab-ci: pre-build containers to improve CI run time and reliability

Shaves about 50% off the build time on both debian and arch builds.
(yeah, I know, it's very small anyway compared to mesa, but we might
 want to add more things in the future)

This also makes the build no longer dependent on external websites:
once the image is build, only fdo-internal services are used.

Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>