title: GitLab CI: How to pull specific docker container for deployment
body: <p>I'm doing the productive deploy in gitlab manually. I'm using docker container. 
  Clicking on the 'Play'-Button in the pipeline list should do the deploy. 
  But how do I get the version of the selected container? Doing this script is always trying to pull the <code>latest</code> version, which should not be. I want to pull the 'selected' container.</p>
  
  <pre><code>deploy_prod:
    stage: deploy
    script:
      - docker pull $CI_REGISTRY_IMAGE     # here selected version is missing
      # ...
    when: manual
    environment:
      name: productive
      url: https://example.com
    only:
      - master
  </code></pre>