Using docker in GitHub repo names is forbidden

They can't do "as they wish". Trademark protection has specific limits, including nominative use ("our product does everything Docker does and more!", "made to work with Docker") and trademarks are specific to a type of product (no conflict between the docker software and a company that makes docks for your phone also called docker, for example).

It seems unlikely that it's legitimate for them to say any repo with "docker" in it is an infringing use, but I think it's often a useful strategy for companies to adopt a broad rule and count on the fact that almost no one will be willing to fight back on this sort of thing because it's much more costly to go to court rather than change the name of your project.