This is problematic, since both uses the "disks_mounted" event to start, and it's almost certain that docker starts first due it's alphabetical precedency.
But, since docker will fail if the image directory doesn't exist, a simple /etc/rc.d/rc.docker start in a script should suffice. I'll elaborate:
Let's assume you mount a disk into e.g. /usr/disks/my_docker_disk and set your Docker image to /usr/disks/my_docker_disk/docker.img.
In case Docker start first, it will fail, since there wouldn't be a /usr/disks/my_docker_disk directory, so you can start it later by writing a script. See the video above.
It works for me, and I see no reason it wouldn't work for you.