Add 'docker.io' and 'index.docker.io' to default registry matcher

Signed-off-by: Michal Fojtik <mfojtik@redhat.com>

pkg/credentialprovider/keyring.go

@@ -106,11 +106,20 @@ const defaultRegistryHost = "index.docker.io/v1/"
 func isDefaultRegistryMatch(image string) bool {
 	parts := strings.SplitN(image, "/", 2)
 
+	if len(parts[0]) == 0 {
+		return false
+	}
+
 	if len(parts) == 1 {
 		// e.g. library/ubuntu
 		return true
 	}
 
+	if parts[0] == "docker.io" || parts[0] == "index.docker.io" {
+		// resolve docker.io/image and index.docker.io/image as default registry
+		return true
+	}
+
 	// From: http://blog.docker.com/2013/07/how-to-use-your-own-registry/
 	// Docker looks for either a “.” (domain separator) or “:” (port separator)
 	// to learn that the first part of the repository name is a location and not