changed tag validation check

Signed-off-by: Xiaoxuan Wang <xiaoxuanwang@microsoft.com>

internal/contentutil/reference.go

@@ -28,5 +28,5 @@ func IsDigest(tagOrDigest string) bool {
 
 // IsValidTag checks if the given string is a valid tag.
 func IsValidTag(tag string) bool {
-	return registry.Reference{Reference: tag}.ValidateReferenceAsTag() == nil
+	return registry.Reference{Reference: tag}.ValidateReferenceAsDigest() != nil
 }