Fix serve command (vercel#3989)

* Fix serve command

From the Firebase docs, you must use --only in order to run the local function emulator.
See https://firebase.google.com/docs/functions/local-emulator

* Add production env for firebase serve

Doesn't work without this

* Update text as suggested by @jthegedus

examples/with-firebase-hosting/README.md

@@ -43,13 +43,9 @@ npm run dev
 
 #### Run Firebase locally for testing:
 
-Unfortunately I have been unable to get any combination of
-
-```bash
-firebase serve --only functions,hosting
 ```
-
-to locally host the built Next.js app as expected. [This issue is where solutions are being explored](https://github.com/firebase/firebase-tools/issues/535) and they will be shared here and on the [Next.js repo's similar issue](https://github.com/zeit/next.js/issues/3167) when discovered.
+npm run serve
+```
 
 #### Deploy it to the cloud with Firebase:
 

examples/with-firebase-hosting/package.json

@@ -4,7 +4,7 @@
   "description": "Host Next.js SSR app on Firebase Cloud Functions with Firebase Hosting redirects.",
   "scripts": {
     "dev": "next src/app",
-    "serve": "echo \"for details see:\n\thttps://github.com/firebase/firebase-tools/issues/535 \n\thttps://github.com/zeit/next.js/issues/3167\";",
+    "serve": "NODE_ENV=production firebase serve --only functions,hosting",
     "deploy": "firebase deploy",
     "clean": "rimraf \"dist/functions\" && rimraf \"dist/public\"",
     "build-public": "cpx \"src/public/**/*.*\" \"dist/public\" -C",