Forking to be seperate from T287913: Enable extension-coverage on most WMF deployed and all MW bundled extensions... As I don't know if we can just apply extension-coverage to all skins, or if we need to do more prep work? Creating skin versions of "mwext-phpunit-coverage-docker-publish" and "mwext-phpunit-coverage-patch-docker", and then putting them under a skin-coverage label?
And we probably want to add something similar to https://doc.wikimedia.org/cover-extensions/ for skins too?