extended clean-up task to remove dangling Docker images

lib/tasks/docker.rake

@@ -2,6 +2,7 @@ namespace :docker do
   desc 'Remove all Docker containers (using the CLI)'
   task :clean_up do
     `docker rm $(docker ps --all --quiet)`
+    `docker rmi $(docker images --filter dangling=true --quiet)`
   end
 
   desc 'List all installed Docker images'