Update soup to prune in background

salt/manager/tools/sbin/soup

@@ -434,8 +434,8 @@ post_to_2.4.10() {
 }
 
 post_to_2.4.20() {
-  echo "Pruning unused volumes"
-  docker volume prune -f
+  echo "Pruning unused docker volumes on all nodes - This process will run in the background."
+  salt --async \* cmd.run "docker volume prune -f"
   POSTVERSION=2.4.20
 }