Don't require rootfs if not set on container

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

image.go

@@ -3,6 +3,7 @@ package containerd
 import "github.com/containerd/containerd/images"
 
 type Image interface {
+	Name() string
 }
 
 var _ = (Image)(&image{})
@@ -12,3 +13,7 @@ type image struct {
 
 	i images.Image
 }
+
+func (i *image) Name() string {
+	return i.i.Name
+}