the question is: is this just a “missing beta feature” or is it “missing by design”?
I hope not to find more workarounds in the near future but port 2375 being exported per default 
P.S: found this thread [worked around] Docker HTTP(S) API no longer available in beta9? via https://github.com/bmuschko/gradle-docker-plugin/issues/235 and related docker-java issues that read like “we won’t fix that, use the damn socket” 
P.S: the #known-issues do list this “solutions” too: