gitlab-ci: Re-use docker image from the main repo in forked repos

gitlab-ci: Re-use docker image from the main repo in forked repos

Instead of generating it from scratch in each forked repo. This should
save time, energy and storage. (The xserver & xf86-video-amdgpu CI
scripts do basically the same)

v2:
* Hardcode "mesa" instead of using $CI_PROJECT_NAME, to avoid breakage
  if the project name is changed after forking (Eric Engestrom)

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>