read environment variable port with fallback to 4000

src/index.ts

@@ -26,7 +26,7 @@ async function main() {
 
   const { url } = await startStandaloneServer(server, {
     listen: {
-      port: 4000,
+      port: process.env.PORT ? parseInt(process.env.PORT) : 4000,
     },
     context: async ({ req, res }) => {
       return {