docker rm -fv $(docker ps -aq)
Sometimes testing produces strange Docker-related problems. The above command kills all containers, including the stopped ones. The "-v" also destroys each container's private storage volumes. This is great because Docker likes to run differently different times you run it -- because it has local state. Local state is evil! Run the above command every now and then so Docker starts from a clean, state-less, state.
In Docker, stopping a container is different from killing it. The former stops the running container/process, but leaves the private volume. The latter zaps the volume.