Use mount.Target to specify subdirectory of rootfs mount

- Add Target to mount.Mount.
- Add UnmountMounts to unmount a list of mounts in reverse order.
- Add UnmountRecursive to unmount deepest mount first for a given target, using
moby/sys/mountinfo.

Signed-off-by: Edgar Lee <edgarhinshunlee@gmail.com>

snapshots/proxy/proxy.go

@@ -226,6 +226,7 @@ func toMounts(mm []*types.Mount) []mount.Mount {
 		mounts[i] = mount.Mount{
 			Type:    m.Type,
 			Source:  m.Source,
+			Target:  m.Target,
 			Options: m.Options,
 		}
 	}