beware, this will be overwritten if you run! docker-machine regenerate-certs <vm-name>
choose a hostpath that suits your needs
VBoxManage sharedfolder add "default" --name "data" --hostpath "/your/host/path/" --automount
VBoxManage setextradata "default" VBoxInternal2/SharedFoldersEnableSymlinksCreate/data 1
docker-machine ssh default
sudo -s
echo "mkdir -p /home/docker/data" >> /var/lib/boot2docker/profile
echo "mount -t vboxsf -o uid=1000,gid=50 data /home/docker/data" >> /var/lib/boot2docker/profile
docker run [...] -v /home/docker/data/another/path:/foo/bar