Error: listen EADDRNOTAVAIL

The interface/IP you asked to bind doesn't exist on this host. 1. Bind to 0.0.0.0 to listen on all interfaces: `app.listen(port, '0.0.0.0')`. 2. Check the IP exists: `ip addr show` / `ifconfig`. 3. In Docker, you cannot bind to the host's LAN IP from inside a container — bind to 0.0.0.0 and map ports with `-p`.