Periodically running docker system prune on a node which regularly runs docker containers is a typical way to prevent unused resources from accumulating forever.
However docker system prune is very drastic in that it deletes _all_ unused resources, even those which are regularly used. It would be useful to have a tool allows the operator to specify high and low water marks for resources of a given type. Instead of deleting everything, it will delete just enough to reach the low water mark. Also, instead of deleting resources indiscriminately, it will delete least recently used resources first.