Reduce disc space requirements for GitLab Runner server
On the Runner machine, each fork of each project gets its own directory,
which is left in the state which the latest job for whatever pipeline was
most recently run on it. This typically will include a large number of
object and binary files, which are of no use to anyone (anything of interest
will already have been packaged up into an artifact and uploaded to the
main GitLab server). Address this by adding an additional job to the end of
each pipeline, which does a git clean (it's worth leaving these in place
to reduce the bandwidth requirement when doing a git fetch when a pipeline
is next run for the fork).
The Runner machine also stores cache files for each fork of each project, at least for jobs that complete fully successfully (and there are an increasing number of these). The way our pipelines use caches, these are tarballs of pre-built source trees for each target platform. These take up less space than the temporary files noted above, but will now become the dominant user of disc space. To address this, abandon use of GitLab Runner's own cache facility, and take advantage of the fact that shell executors actually have visibility of the gitlab-runner user's whole home directory to maintain a single, cached version of each tarball, shared across all forks of all projects. This is stored within ~/cache, but namespaced under ~/cache/common to avoid collisions with any users of GitLab Runner's cache facility.