Personally I would fork it only when really necessary, IMHO it's better to join the forces (which is actually what you already do, if it's not only coincidence of names), maintain and develop one project over to have dozen of forks, keep and eye on them and eventually cherry-pick good and needed changes (from my experience, it's quite time consuming and late response to cherry-pick something could be an issue e.g. in case of security bugs). Let's keep this issue open for a while and see how docker-java will evolve, but for now I would keep it.
FYI: I just realized that https://github.com/jenkinsci/docker-plugin uses jDocker (https://github.com/nirima/jDocker)