boot2docker README mentions
Alternatively, Boot2Docker includes added VirtualBox guest add-ons specifically designed to share the VirtualBox folder.
The first of the following common name names that exist (if any) will be automatically mounted in the specified location:
c/Users share at /c/Users /c/Users share at /c/Users c:/Users share at /c/Users
If any other path or shared resource is required, it can be set at runtime by doing something like:
$ mount -t vboxsf -o uid=1000,gid=50 your-other-share-name /some/mount/location
You can see more in VirtualBox / guest add-ons / shared folders
At the command line, you can create shared folders using VBoxManage, as shown below:
VBoxManage sharedfolder add "boot2docker-vm" --name "sharename" --hostpath "C:\test"
source share