title: Bundle not found when deploying to gcloud
body: <p>I'm deploying a rails app to Google Cloud App Engine. Since adding </p>
  
  <pre><code>config.action_mailer.default_url_options = { host: 'foobar.appspot.com'}
  </code></pre>
  
  <p>to production.rb, the build fails on deploy. Here are the logs:</p>
  
  <pre><code>INFO[0108] RUN bundle install --deployment --without="development test" &amp;&amp; rbenv rehash 
  INFO[0108] cmd: /bin/sh 
  INFO[0108] args: [-c bundle install --deployment --without="development test" &amp;&amp; rbenv rehash] 
  /bin/sh: 1: bundle: not found
  error building image: error building stage: waiting for process to exit: exit status 127
  </code></pre>