Update the reference parsing logic to match what common tools do today (e.g. Docker CLI)

Author: jcarter3

ociref/dockerhub.go

@@ -0,0 +1,9 @@
+package ociref
+
+var dockerHubHosts = map[string]bool{
+	"":                        true,
+	"docker.io":               true,
+	"index.docker.io":         true,
+	"registry-1.docker.io":    true,
+	"registry.hub.docker.com": true,
+}

ociref/reference.go

@@ -195,7 +195,7 @@ func ParseRelative(refStr string) (Reference, error) {
 			return Reference{}, fmt.Errorf("invalid digest %q: %v", ref.Digest, err)
 		}
 	}
-	if ref.Host == "" {
+	if dockerHubHosts[ref.Host] {
 		ref.Host = "docker.io"
 	}
 	if len(ref.Repository) > 255 {