GitHub pages is essentially a closed box - GitHub don't tell us what versions of Jekylll, Liquid, KramDown etc. they are running.
An then again, they actually do!
The pages-gem for Ruby is maintained by GitHub and is at all times in sync with the version running on GitHub pages.
The main purpose of this Docker image here, is to provide a Docker image, that resembles the GitHub pages. By using this immage to test your pages, it should be fair to expect that they will behave the same when published to GitHub pages.
The image is maintained by Praqma