CI runner does not cache docker builds
This is a problem with GitLab that has been vexing me: https://gitlab.com/gitlab-org/gitlab-ce/issues/17861
There is a solution -- mount the host docker socket rather than using the docker-in-docker
service, but it comes at the price of reduced concurrency. Once the new build system goes in we will have to decide what we want to do about this. GitLab are working on it in the meantime.