duneci-docker-gc: Cleanup left-overs from Docker.

Docker keeps containers and images around after the container is no
longer in use. These eat up all disk space and need to be cleaned up
from time to time.

bin/duneci-docker-gc

+#!/bin/bash
+#
+# Clean up left-overs from Docker.
+
+set -e
+set -u
+
+docker rm $(docker ps -a -q -f status=exited)
+docker rmi $(docker images --filter dangling=true -q)