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>

services/diff/local.go

@@ -171,6 +171,7 @@ func toMounts(apim []*types.Mount) []mount.Mount {
 		mounts[i] = mount.Mount{
 			Type:    m.Type,
 			Source:  m.Source,
+			Target:  m.Target,
 			Options: m.Options,
 		}
 	}

services/snapshots/service.go

@@ -310,6 +310,7 @@ func fromMounts(mounts []mount.Mount) []*types.Mount {
 		out[i] = &types.Mount{
 			Type:    m.Type,
 			Source:  m.Source,
+			Target:  m.Target,
 			Options: m.Options,
 		}
 	}

services/tasks/local.go

@@ -215,6 +215,7 @@ func (l *local) Create(ctx context.Context, r *api.CreateTaskRequest, _ ...grpc.
 		opts.Rootfs = append(opts.Rootfs, mount.Mount{
 			Type:    m.Type,
 			Source:  m.Source,
+			Target:  m.Target,
 			Options: m.Options,
 		})
 	}