Connect players to a fallback server when main backend server is down

I'm currently using a setup very similar to the one on the Docker Compose Usage example for our survival server.

I'm wondering if there is a way to configure a fallback server to connect players to when the main server is down. Mainly to show an MOTD message, but also so we still have a smaller server to hang around in to wait for the main server host to come back up.

If this is not yet a feature then I think it would be very useful to have.

No source file or test is named. Start with the Docker Compose Usage example and the router's existing backend connection and configuration behavior; define how a fallback server is selected when the main server is unavailable, including the MOTD and player-routing expectations, then add coverage for the failure and recovery cases.

Written by the indexing model from the issue text.