They've always had the functionality to run software like Transmission inside a Jail. I guess they just moved that functionality to docker to be more "hip".
Reminds me of the joke, "How do you get docker to be secure? Run it inside a (FreeBSD) jail."
Having not looked at it that's my guess what they are doing. Everything related to it is in a jail. Making it secure regardless of dockers questionable security.
Of course jails are the underlying mechanism of that Docker port on FreeBSD that someone did once⦠But I'm not sure they're using that. Linuxulator is not perfect, and they announced "Docker support" not "imperfect Docker support" :D I bet they're using something like boot2docker in a bhyve VM.