The /cache volume mount accumulates gitlab runner cache
files. This is useless due to the second level distributed
S3 minio cache which is localhost on metal runners.

Drop this volume mount to safe some RAM.

Note cache files are now downloaded each time and later rot
in the main builds_dir volume mount until next job performs
a git clean in an early preparation step. But it's only
one file now, not multiple.

It would be even better if we could 'git clean' the build_dir
after job execution and after (!) cache upload as one very last
step to not have the .cache/ dir in builds_dir after job
execution at all (and other stuff created by job), but gitlab
currently provides no way to do that (?!)