@dbarratt stated he doesn't have much extra time these days so we are recruiting someone to replace him and join myself and @Addshore.
The main responsibility of a maintainer is to update the mediawiki-docker repository following a release (e.g. https://github.com/wikimedia/mediawiki-docker/pull/118) and then submit it to the docker-library folks for pushing to docker hub (e.g. https://github.com/docker-library/official-images/pull/13792). And then triage and address issues in Docker-Hub-MediaWiki as appropriate.
Ideally the maintainer would be already trusted in the MediaWiki community and probably have +2 rights somewhere.