Some hacking has revealed:
cd ~/Library/Containers/com.docker.docker/Data/database/com.docker.driver.amd64-linux
echo '{"storage-driver":"aufs","dns":["8.8.8.8"],"insecure-registries":["yourdomain.com:5000"],"debug":true}' > etc/docker/daemon.json
git add etc/docker/daemon.json;
git commit -m 'etc/docker/daemon.json change';
The git commit will trigger an xhyve VM restart (git commit hook?). The Docker for Mac menulet icon in OS X will show a restart.
The Docker for Mac preferences will now show yourdomain.com:5000 as an insecure registry.
docker run -it --rm busybox cat /etc/resolv.conf will show that the nameserver for containers is now 8.8.8.8