For docker on freebsd to get any traction, it would need to be implemented with the vm behind the scenes. The beauty of the macOS/windows versions are that they require 0 setup/maintenance and are an implementation detail.
It seems like this should be relatively simple; build docker-cli as a native FreeBSD program, make a docker-machine backend for bhyve... port docker-machine to FreeBSD, I guess (I hope that's not hard; it's not doing anything OS-specific), and you should be good to go.