You can search for that error and get some ideas.
Your OS is out of date. Your sources.list is out of date. You need to upgrade your server. I would recommend that you move to a new one.
Itās possible that you have a new OS (but thatās unlikely since thatās a very old Docker, circa 2021) and that somehow something messed up that file.
If you donāt want to upgrade your OS, you donāt want to upgrade Discourse.