Clients fail to connect to cluster mode processes:
socket.io.js:4026 WebSocket connection to 'ws://localhost:3000/socket.io/?EIO=3&transport=websocket&sid=UBuQ9h7ht3Nu5zXZAAAD' failed: Connection closed before receiving a handshake response
Fork mode processes couldn't reuse the same port.
PM2 exposes process.env.NODE_APP_INSTANCE for the rescue.
.listen(Number(PORT) + Number(NODE_APP_INSTANCE))