Using Docker - No plugins required Last time I investigated using Docker inside the “master” container. It was pretty cool, but it was inconvenient when directories needed to be mounted to the containers.
Avoid Docker-in-Docker Using Docker-in-Docker for your CI or testing environment? Think twice. by @jpetazzo explains why the DinD approach is bad for CI, and how using an alternative approach will allow the docker container to create sibling containers rather than child containers.
Clean up, before I get messy I have been running a lot of different docker containers, and often forgetting to use --rm. So I pruned my system: docker system --prune as recommended by this answer on StackOverfow.