Update CI Docker docs
What does this MR do?
Updates documentation with:
- instructions for bind-mounting for docker runners (fixes #17769 (closed) and #13898 (closed))
- examples of using the GitLab Container Registry in docker-based builds (fixes #17968 (closed) and #17967 (closed))
- update runner instructions for
gitlab-ci-multi-runner(fixes https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/1337) - some grammar fixes
Are there points in the code the reviewer needs to double check?
The markdown in general. The runner commands. I haven't been able to verify the socket version works yet.
Why was this MR needed?
Better documentation for users.
What are the relevant issue numbers?
#17769 (closed), #13898 (closed), #17968 (closed), #17967 (closed)