title: React app on App Engine not finding static files with custom domain
body: <p>I deployed my app to App Engine and it works with the default subdomain that is auto generated. However, after going through the steps in the mapping a custom domain section of the docs I can't see it at the custom domain.</p>
  
  <p>here's my app.yaml:</p>
  
  <pre><code>runtime: nodejs10
  
  service: platform
  
  automatic_scaling:
    max_instances: 1
  
  handlers:
    - url: /static/js/(.*)
      static_files: build/static/js/\1
      upload: build/static/js/(.*)
    - url: /static/css/(.*)
      static_files: build/static/css/\1
      upload: build/static/css/(.*)
    - url: /static/media/(.*)
      static_files: build/static/media/\1
      upload: build/static/media/(.*)
    - url: /(.*\.(json|ico))$
      static_files: build/\1
      upload: build/.*\.(json|ico)$
    - url: /
      static_files: build/index.html
      upload: build/index.html
    - url: /.*
      static_files: build/index.html
      upload: build/index.html
  </code></pre>
  
  <p>Here are the errors I'm getting:
  <a href="https://i.stack.imgur.com/uCLD1.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/uCLD1.png" alt="enter image description here"></a></p>