title: Error while following GKE tutorial to run rails on GKE
body: <p>While following <a href="https://cloud.google.com/solutions/migrating-ruby-on-rails-apps-on-heroku-to-gke" rel="nofollow noreferrer">https://cloud.google.com/solutions/migrating-ruby-on-rails-apps-on-heroku-to-gke</a> I get the error:</p>
  <pre><code>&quot;/usr/local/lib/ruby/2.6.0/rubygems.rb:283:in find_spec_for_exe': Could not find 'bundler' (2.2.15) required by your /usr/src/app/Gemfile.lock. (Gem::GemNotFoundException)
  To update to the latest version installed on your system, run `bundle update --bundler`.
  To install the missing version, run `gem install bundler:2.2.15`
          from /usr/local/lib/ruby/2.6.0/rubygems.rb:302:in `activate_bin_path'
          from /usr/local/bin/bundle:23:in `&lt;main&gt;'&quot;
  </code></pre>
  <p>when I try to run: <code>docker build -t ruby-app .</code></p>
  <p>Anybody know what's wrong?</p>