The author shows a concrete exploit of the kernel bug described in CVE-2017-5123 as he has developed it in the context of the docker container environment.
He shows how to use this bug to break out of docker, so he calls the blog post "Escaping Docker ...".
Which is IMHO the most interesting container runtime to write such an exploit for first because it is very widely deployed, but it might also just have been what the author is most familiar with or what was easiest to develop for him.