More days later...
Update: many of the bugs and glitches I mention below have been fixed in Docker version 8.
I was able to access the FMS server running in the VirtualBox image on my MBA but only from within the image. Couldn't access it from my browser running on my MBA.
I tried a variety of hacks. The first one was to ignore dvm/vagrant and use VB's console to change the networking setting for Adapter 1 to bridge => en2: Display Ethernet. This allowed me to set the IP address to one within my LAN and I was able to access FMS in my browser.
But this required manual intervention and I wanted the image to be as automatic as possible. So I tried editting ~/.dvm/Vagrantfile to created a public network and to set the adapter to bridge. But this caused a conflict error message to halt the startup. I tried a variety of changes and none remove the conflict.
Eventually I realised that the default setting (192.168.42.43) address is accessible to the browser running on my MBA. Not sure why I didn't check this first. Unfortunately it's not accessible by other browsers on my LAN (192.168.1.*) but it's only for development so I'm happy.
Then I used Intercity's note about flattening the Docker image. It dropped from approx. 2GB down to 750MB. Still quite large but better.
Then I tried to push the image to Docker's public repository. And I'm still trying... The upload keeps timing out, or the Docker DNS's disappear or something else.
Intercity's flattening note also shows how to export the image to a tarball and reimport it and I've at least got a tarball with which I can re-import the image as needed.
So lots of "learning experiences". I still think Docker might be the best devel tool since the cloud appeared but still many rough edges for people who use Macs.