I/O errors, nice. What kind of disk is this? Removable USB case? Fixed one? Are you able to test the disk in another case or directly plugged into a mainboard?
In addition to the explanation you got from the other user: once you've set up the bouncer middleware in the configs (don't know if there even exists a good way to do that outside of the configs files), you simply assign the middleware in the compose file as usual.
Realistically, most self-hosted services will work "offline" (no internet connection), provided you also power a local network during the outage as you will have to access it somehow.
This also plays into the other factors, as self-produced power is usually far less than what you'd get from the grid (I.e. you'll probably want to avoid things like heavy transcoding).
That's how it works on Reddit, which is likely the only forum-like website many users are used to.