title: Modify a service inside GitLab CI
body: <p>I'm attempting to set up GitLab CI and I have some integration tests that run against elasticsearch. I'd like to install elasticsearch using the official docker image, so:</p>
  
  <pre><code>services:
    - elasticsearch:2.2.2
  </code></pre>
  
  <p>But I want the mapper-attachments plugin. I have had no luck adding a command in the <code>before_script</code> section to install the mapper-attachments plugin, because the elasticsearch files don't seem to be in the environment that the before_script section is running inside of. How can I modify the elasticsearch image that has been installed into the runner? </p>