Thanks philwhln, I'd understood that much :) I guess I'm asking: What would be the selling point of using Docker (LXC) over Jails/Zones?
You can run the full-stack inside a container. eg. run Red Hat or CentOS inside Ubuntu. The process inside LXC / Docker is unaware it's inside a container. You can have 100 processes all think they're binding to a given port, which simplifies things within the container. This is why docker images work so well: they can be fired up and use the same configuration no matter how many instances you create. LXC = jail on steroids.