title: Bundler can't see gems installed in dockerized environment
body: <p>I have a pretty basic "app" which i'm trying to dockerize using some examples found on the internet but, for some reason, it seems that bundler can't see the gems installed in my env.</p>
  
  <h3>Gemfile</h3>
  
  <pre><code>source 'https://rubygems.org'
  gem 'streamio-ffmpeg'
  </code></pre>
  
  <h3>Dockerfile</h3>
  
  <pre><code>FROM dpsxp/ffmpeg-with-ruby:2.2.1
  MAINTAINER mike
  
  RUN apt-get update &amp;&amp; apt-get install -y \
    build-essential \
    cmake
  
  ENV APP_HOME=/app
  
  RUN mkdir -p $APP_HOME
  WORKDIR $APP_HOME
  
  COPY Gemfile* ./
  ENV BUNDLE_GEMFILE=$APP_HOME/Gemfile \
    BUNDLE_PATH=/bundle
  RUN gem install bundler
  RUN bundle install --jobs 20 --retry 5 --path=/bundle
  
  COPY . ./
  
  CMD ["rake"]
  </code></pre>
  
  <h3>docker-compose.yml</h3>
  
  <pre><code>version: '2'
  
  services:
    app:
      build:
        context: .
      volumes:
        - '.:/app'
      depends_on:
        - bundle
  
    bundle:
      image: busybox
      volumes:
        - /bundle
  </code></pre>
  
  <p>And now, when i'm running <code>$ docker-compose run app gem list</code>, the <code>streamio-ffmpeg</code> gem is not on the list for some reason. Also, when i'm trying to require it inside my ruby app, i'm getting <code>LoadError: cannot load such file</code></p>
  
  <p>Build logs: <a href="https://gist.github.com/mbajur/569b4f221cadb8a85ecc5dae8a595401" rel="nofollow noreferrer">https://gist.github.com/mbajur/569b4f221cadb8a85ecc5dae8a595401</a></p>