This was a feature I removed in 47c090be ("executor/gitlab_runner_cfg:
drop the now-unused /cache volume") because we did not use it, and also
because it would never have been doing anything useful when it was introduced
as the gitlab runner was running in a container itself... so /cache would be
fresh every single time.

This time around, I decided to use a volume to back /cache. This way, no nasty
surprises about privileges whenever we'll move the gitlab runner to be running
as a user.