duneci-docker-gc: Use "docker rm -v"

Without "-v", docker would keep around volumes used by the container
forever and eventually fill the disk.

bin/duneci-docker-gc

@@ -5,5 +5,5 @@
 set -e
 set -u
 
-docker rm $(docker ps -a -q -f status=exited)
+docker rm -v $(docker ps -a -q -f status=exited)
 docker rmi $(docker images --filter dangling=true -q)