WIP: Update docker gitlab-runner docs

What does this MR do?

See the general guidelines: http://docs.gitlab.com/ce/development/doc_styleguide.html

Moving docs to a new location?

See the guidelines: http://docs.gitlab.com/ce/development/doc_styleguide.html#changing-document-location

  • Make sure the old link is not removed and has its contents replaced with a link to the new location.
  • Make sure internal links pointing to the document in question are not broken.
  • Search and replace any links referring to old docs in GitLab Runner's source code