I love Docker, it's a fantastic concept, and so far the execution and progress of the project has been flawless. I also love FreeBSD; FreeBSD is a clean and powerful system with advanced features like Dtrace, ZFS, and Jails. Combine the two and it sounds better than chocolate and peanut butter. With the recent version 0.9 release, Docker announced the infrastructure support to glue the two together, along with KVM, OpenVZ, Solaris Zones, and nearly any other environment for application isolation through an execution driver API.
The execution driver API support allows third parties to extend Docker to customize the environment of the container Docker builds. From what I can tell, the FreeBSD execution driver hasn't been built yet, but I imagine it is not too far away. I envision that the (admittedly imaginary) FreeBSD driver would also take advantage of ZFS and the ability to create snapshots, clones, and the other ZFS goodness. The Docker FreeBSD driver could take the setup and possible complexity out of building and releasing FreeBSD Jails, opening up powerful technology to a wider audience.