The boot2docker README mentions
Alternatively, Boot2Docker includes the VirtualBox Guest Additions built in for the express purpose of using VirtualBox folder sharing.
The first of the following share names that exists (if any) will be automatically mounted at the location specified:
c/Users share at /c/Users
/c/Users share at /c/Users
c:/Users share at /c/Users
If some other path or share is desired, it can be mounted at run time by doing something like:
$ mount -t vboxsf -o uid=1000,gid=50 your-other-share-name /some/mount/location
You can see more at VirtualBox / Guest Additions / Shared folders
From the command line, you can create shared folders using VBoxManage, as follows:
VBoxManage sharedfolder add "boot2docker-vm" --name "sharename" --hostpath "C:\test"