Added docker run gitlab-runner examples

Hi. I'm trying to use gitlab-ci with docker excutor. I've been confused that how I could make runner containers without editing config.toml manually(You know all docker people love the sound of automating 😃 ). I guess there are no description in those docs so that I hope you accept my examples in the doc.

And I'd like to need other things;

1. It's useful if the official doc has links to these docs in here. Are there some reasons it doesn't have those?
2. I'd like to see gitlab-runner exec docker --help response on internet and official doc (It was very hard to find. I accidently found it with typo 😖 ). Is there a good way to add the file somewhere and keep updating it?