@imc67 Ah yes, we used to call this "retire" back in the managed hosting days 🙂 We used to show a page that the server is migrating. Incidentally, I just removed the retire script last night! It's really just systemctl stop docker box but of course these days you will see a "app is down page" instead of a better "migration in progress" page.
https://d2c.io is pretty decent. Not open-source but self-hosted and does clustering well. They have a Discord channel I nudged them to setup but it only gets a post every few months. Decent project though and some solid app-stacks packaged for one-click installs.