At least one Tails' GitLab Runner is storing cache data in the host's /cache directory

At least one Tails' GitLab Runner is storing cache data in the host's /cache directory and this shouldn't be happening, i expected all the cache data to go into /var/local/gitlab-runner-cache/.

Note that there are 2 levels of cache in GitLab Runner Docker executors:

• GitLab Runner build cache: this seems to be wrongly landing in /cache
• Docker executor cache: this seems to be correctly landing in /var/local/gitlab-runner-cache

We currently don't set runners.cache_dir, but do set runners.docker.cache_dir="/cache" for one of our runners.

I find the upstream doc a bit confusing, so there may be an upstream bug there, either in the doc itself or in the config parsing code.