They'd probably need to do both, as users who want to use docker depend on very specific parts of the docker ecosystem (docker hub, the docker daemon itself, etc).
Not to mention this is essentially just packaging, I hope they package any software they find reasonably popular and let users decide what's more important to them.
The link sends you to the packaging, but this is a bonafide port which uses FreeBSD jails. The code is not yet upstream in Docker, but I imagine (hope) we'll see this work come in as pull-requests.
(Disclosure: I am an employee of Docker, Inc. Any opinions expressed here are my own)
Not to mention this is essentially just packaging, I hope they package any software they find reasonably popular and let users decide what's more important to them.