And the message that “You are running an old version of the Discourse image” doesn’t answer your question? If it said “You are running an old version of the Discourse Docker image” would that be any more clear?
As you noted, you’re the the first to be confused by this message, but I don’t know what else it could say.
The Docker image has a whole bunch of stuff in it (nginx, ruby, rails, for starters) and sometimes it needs to be updated.