Update Docker installation method docs
What does this MR do?
- Updates the information about
gitlab/gitlab-runner
base image for the Ubuntu version of the image (we're using Ubuntu 16.04 since few weeks). - Adds a description about general usage of Runner's images as a replacement of a locally executed
gitlab-runner
command. This is an usable addition that came up from #3266 (closed) and #3267 (closed).
Closes #3267 (closed)
Edited by Tomasz Maczukin