Add 'ImageLayoutFile' const

The image layout MUST contain an `oci-layout` file.
Add a const to ref this, so the image tool can use
this to validate.

Signed-off-by: Lei Jitang <[email protected]>

Author: coolljt0725

specs-go/v1/layout.go

@@ -17,7 +17,10 @@ package v1
 import "regexp"
 
 // ImageLayoutVersion is the version of ImageLayout
-const ImageLayoutVersion = "1.0.0"
+const (
+	ImageLayoutFile    = "oci-layout"
+	ImageLayoutVersion = "1.0.0"
+)
 
 // ImageLayout is the structure in the "oci-layout" file, found in the root
 // of an OCI Image-layout directory.