title: How to bundle install a local path gem with docker?
body: <p>I am using docker and rails.</p>
  
  <p>I created a local gem and put it into vendor/gems folder. </p>
  
  <p>And I add this into my Gemfile:</p>
  
  <pre><code>gem 'my_gem', path: './vendor/gems/my_gem'
  </code></pre>
  
  <p>In my Dockerfile</p>
  
  <pre><code>RUN mkdir /testapp
  WORKDIR /testapp
  ADD Gemfile /testapp/Gemfile
  ADD Gemfile.lock /testapp/Gemfile.lock
  RUN bundle install
  ADD . /testapp
  </code></pre>
  
  <p>After run <code>docker-compose build</code>, it shows:</p>
  
  <pre><code>The path `/testapp/vendor/gems/my_gem` does not exist.
  ERROR: Service 'web' failed to build: The command '/bin/sh -c bundle install' returned a non-zero code: 13
  </code></pre>