docker creates a bunch of files when it builds a container (include the containers that it builds) when you build a new container, you still have the old containers (and also disk images). That’s what gets removed when you do a cleanup.
It’s a little bit safer to run the cleanup command while discourse is running, because the cleanup command won’t delete a running container. If something goes wrong with the rebuild, you can still restart the old container if it exists.
So, I’d remove step (1) above and step (3) is unnecessary because ./launcher does a pull (back in the old days, it didn’t).