Add SNAPCAST_REMOTE_HOST as exemplary environment variable as it is mandatory for snapcast client to work

[oerkel47]

This should make first setup more obvious. I was wondering why the container was unhealthy, checked logs and found this

2026-05-15 12-57-40.503 [Info] (Controller) Reconnecting
2026-05-15 12-57-41.503 [Info] (Connection) Resolving host IP for: 192.168.33.5
2026-05-15 12-57-41.503 [Info] (Connection) Connecting to host: 192.168.33.5:1704, port: 1704, protocol: tcp
2026-05-15 12-59-55.670 [Error] (Connection) Failed to connect to host '192.168.33.5', error: Connection timed out
2026-05-15 12-59-55.670 [Error] (Connection) Error in socket shutdown: Transport endpoint is not connected

I was wondering where this seemingly random IP came from and found it hard coded in the .env file. I think it makes sense to set "mandatory" user variables inside the compose file and add this example to make it more obvious.