title: Create-React-App Proxy in Production Build
body: <p>I'm using create-react-app with an express backend. I have the backend running on port 3001, and the frontend runs through port 3000 when in dev mode. I have <code>"proxy": "http://localhost:3001"</code> in my package.json and the api works perfectly.</p>
  
  <p>When I use yarn build, however, and then run <code>serve -s build</code>, the api calls are simply not being made. I'm not sure why it doesn't work in production, when it does in dev.</p>
  
  <p>Any help would be much appreciated.</p>