Add option to make a filesystem path an overlay over the real contents

I'd like to be able to create multiple containers to work on the same project in parallel without affecting each other, e.g. working on a new git branch while other branch is building.

I guess this can be done by running `fuse-overlayfs` manually in each container but it'd be great if `toolbox create` offered an option like: `--unshare-path=` or similar that allowed to work on top of the path without changing the underlying file system.