Clean up Docker artifacts in dev environments

Remote dev environments that get continuous Docker deployments should have their images and volumes cleaned up regularly.

Pass the following commands over SSH to the remote server, perhaps in a Jenkins job for example:

 sudo -- bash -c 'docker volume rm $(docker volume ls -f dangling=true -q)' > /dev/null 2>&1  
 sudo -- bash -c 'docker images --quiet --filter=dangling=true | xargs --no-run-if-empty docker rmi' > /dev/null 2>&1  
 echo $?  

echo $?  .. is used at the end because if the images are already cleaned up there may be an exit status code indicating and error .. which can actually safely be ignored so echo $? will evaluate it so that Jenkins will not report the job as failed.