WIP 'docker share' using rsync or sshfs - only does 'pwd'

Thought I'd just try it :)

The `--share=smb` share driver flag does not work - we need to merge #245 and some docker/pkg/mflags changes (or #243 and and some more fixes)
if we were actually going to release this, we'd have to fix the many layers of races by mounting the dir's into hashed dirs. You need to avoid clashes with other users', clashes with shares from other computers, and all sorts of stuff.