title: does appengine cloudbuild.yaml requires a custom runtime?
body: <p>Build errors out with below output (Using a Rails app)</p>
  
  <pre><code>ERROR: (gcloud.app.deploy) There is a cloudbuild.yaml in the current directory, and the runtime field in /workspace/app.yaml is currently set to [runtime: ruby]. To use your cloudbuild.yaml to build a custom runtime, set the runtime field to [runtime: custom]. To continue using the [ruby] runtime, please remove the cloudbuild.yaml from this directory.
  </code></pre>