title: Multiple Docker images in .gitlab-ci.yml
body: <p>Here is my problem setup with GitLab and its integrated CI service. I have a current GitLab 8.1. and a gitlabci-multi-runner (0.6.2) with Docker support. After extending the ubuntu:precise image to include <code>git</code> and <code>build-essentials</code> (now named <em>precise:base</em>) I got the following <code>.gitlab-ci.yml</code> running:</p>
  
  <pre><code>image: precise:base
  before_script:
     - apt-get install --yes cmake libmatio-dev libblas-dev libsqlite3-dev libcurl4-openssl-dev
     - apt-get install --yes libarchive-dev liblzma-dev
  
  build:
    script:
      - mkdir build/
      - cd build
      - cmake -D CMAKE_BUILD_TYPE=Debug ../
      - make
  </code></pre>
  
  <p>Now my question is how to include more jobs on different images? Because I need to check if the code compiles (and later on works) on different operating systems like Ubuntu Precise, Ubuntu Trusty, CentOS 6, CentOS 7. To reduce the work I think the best way is to provide different Docker images as base.  </p>
  
  <p>Now the questions is how must the <code>.gitlab-ci.yml</code> look like to support this? </p>