remove Dockerfile dependency on labs/lxc-gitlab-runner

The repository labs/lxc-gitlab-runner is pretty much obsolete. Admins are not using it, they have copied relevant code to their repositories. What remains are the container definitions. I suggest including the Dockerfiles from there into ci/lxc here and ditching that repository completely. The cross-repo dependency between Dockerfiles is pretty fragile and it's IMO just a matter of time until something breaks, because someone won't notice the dependency. (Actually, it happened to me recently and I've spent several hours debugging the problem...)