title: How to set build .env variables when running create-react-app build script?
body: <p>I'm using the following environment variable in my create-react-app:</p>
  
  <pre><code>console.log(process.env.REACT_APP_API_URL) // http://localhost:5555
  </code></pre>
  
  <p>It works when I run <code>npm start</code> by reading a <code>.env</code> file:</p>
  
  <pre><code>REACT_APP_API_URL=http://localhost:5555
  </code></pre>
  
  <p>How do I set a different value like <code>http://localhost:1234</code> when executing a <code>npm run build</code>?</p>
  
  <p>This is my <code>package.json</code> file:</p>
  
  <pre><code>{
    "name": "webapp",
    "version": "0.1.0",
    "private": true,
    "devDependencies": {
      "react-scripts": "0.9.0"
    },
    "dependencies": {
      "react": "^15.4.2",
      "react-dom": "^15.4.2"
    },
    "scripts": {
      "start": "react-scripts start",
      "build": "react-scripts build",
      "test": "react-scripts test --env=jsdom",
      "eject": "react-scripts eject"
    }
  }
  </code></pre>