2018 Update: Easiest option is Justin's repo and image
Just run this from your Mac terminal and it'll drop you in a container with full permissions on the Moby VM. This also works for Docker for Windows for getting in Moby Linux VM (doesn't work for Windows Containers).
docker run -it --rm --privileged --pid=host justincormack/nsenter1
more info: https://github.com/justincormack/nsenter1