title: Gitlab CI Build failed gitlab-runner-prebuilt.tar.xz: no such file or directory
body: <p>I just installed Gitlab Runner on my dev machine (Ubuntu 17.10) for testing.
  When I run the runner I got:</p>
  
  <pre><code>$: sudo gitlab-runner exec docker test       
     Running with gitlab-ci-multi-runner dev (1.4.2)
     Using Docker executor with image php:5.6 ...
     ERROR: Build failed (system failure): open /var/lib/gitlab-runner/gitlab-runner-prebuilt.tar.xz: no such file or directory
     FATAL: open /var/lib/gitlab-runner/gitlab-runner-prebuilt.tar.xz: no such file or directory 
  </code></pre>
  
  <p>.gitlab-ci.yml file:</p>
  
  <pre><code>image: php:5.6
  
  before_script:
    - php -v
  
  stages:
    - test
  
  test:
    script:
    - php -v
  </code></pre>
  
  <p>Current installation process:</p>
  
  <pre><code>sudo apt-get install gitlab-runner
  </code></pre>
  
  <p>Output:</p>
  
  <pre><code>...
  Configuring gitlab-ci-multi-runner (1.4.2+dfsg-1) ...
  I: generating GitLab Runner Docker image. This may take a while...
  E: No mirror specified and no default available
  W: please run 'sudo /usr/lib/gitlab-runner/mk-prebuilt-images.sh' to generate Docker image.
  ...
  </code></pre>
  
  <p>So I did:</p>
  
  <pre><code>$: sudo /usr/lib/gitlab-runner/mk-prebuilt-images.sh
     I: generating GitLab Runner Docker image. This may take a while...
     E: No mirror specified and no default available
  </code></pre>