Move dokku cleanup into docker_cleanup function

This should slightly reduce DOKKU_TRACE output.

plugins/common/functions

@@ -320,3 +320,12 @@ release_and_deploy() {
     echo
   fi
 }
+
+docker_cleanup() {
+  # delete all non-running container
+  # shellcheck disable=SC2046
+  docker rm $(docker ps -a -f 'status=exited' -q) &> /dev/null || true
+  # delete unused images
+  # shellcheck disable=SC2046
+  docker rmi $(docker images -f 'dangling=true' -q) &> /dev/null &
+}