Embed docker images used by runner in runner
We used the gitlab/gitlab-runner:{build,cache,service}
that were fetched from Docker Hub.
Changes:
This MR adds this images as static assets to runner.
This MR also introduced gitlab-runner-helper
that is used by these images.
How it works: Runner when on the first use will upload them to Docker.
Why?
- Make the runner simpler to version: the images are tied to revision of build.
- Make the build faster to start.
- Make the runner easier to use behind corporate firewall.