New runner and docker recipes

[chewi]

We're setting up some CI stuff here. We haven't put it into production yet but thought I'd put this out there now anyhow. There's a lot less to these recipes than I thought there would be, thanks largely to the packagecloud and docker cookbooks.

We're using docker-compose as well but I gather that isn't the standard approach under GitLab CI and our own recipe for installing that does nothing GitLab-related anyway.

[chewi]

Just changed it to use the "default" Docker service instance of a "gitlab" instance. The per-instance stuff seems a bit broken and it's just counter-productive in this case.