How to run a CRA in Heroku w/ Express and Node.js build pack

package.json

/* info: the original "start" from CRA has been renamed to dev and a new "start" that points to server.js has been added */
{
  "scripts": {
    "start": "node server.js",
    "dev": "react-scripts start",
    "build": "react-scripts build",
    "test": "react-scripts test --env=jsdom",
    "eject": "react-scripts eject"
  }
}

server.js

/* 
 * author: Mattia Richetto
 * twitter: @mattiaerre
 * email: mattia.richetto@gmail.com
 * date: 12/10/2016
 */

const express = require('express');

const app = express();

app.use(express.static('build'));

const port = process.env.PORT || 3000;

app.listen(port);