title: Gitlab-runner docker executor
body: <p>I am using gitlab-runner into a docker container. I'd like to run my builds into a docker container that would embed any depedencies needed. 
  What kind of executor should I  use? docker ? If I do that, I run builds on a nested container which is not that recommended I guess.</p>
  
  <p>What is the best practice?
  thanks</p>