Set up cache for GitLab CI runners

The GitLab CI runners are on-demand Docker containers running on a DigitalOcean host at IP 138.62.143.2.

Because they are created on-demand there is no data persisted between jobs. This means that all the sources and cached artifacts required for the build need to be redownloaded from git.baserock.org and cache.baserock.org every time.

Setting up a local cache server following these instructions would save on bandwidth and speed up builds.