Docker requires a bit of setup before usage. 

You will need to create a ZFS dataset on /usr/docker

# zfs create -o mountpoint=/usr/docker <zroot>/docker 

And lastly enable the docker daemon
# sysrc -f /etc/rc.conf docker_enable="YES"
# service docker start

(WARNING)

Starting the docker service will also add the following PF rule:

nat on ${iface} from 172.17.0.0/16 to any -> (${iface})

Where $iface is the default NIC on the system, or the value
of $docker_nat_iface. This is for network connectivity to docker
containers in this early port. This should not be needed in future
versions of docker.