As long as there’s a simple way to determine which containers use outdated images, I’m good

Yeah you can either have it update the containers itself, or just print out their names. With a custom plugin you can make it output the names of any containers that have available updates in whatever format you like. This discussion on the github page goes through some example scripts you can use to serve the list of containers with available updates over a REST API to be pulled into any other system you like (eg: Homepage dashboard).

github.com/mag37/dockcheck/discussions/146