add http head based digest comparison to avoid dockerhub rate limits

pkg/registry/helpers/helpers.go

@@ -0,0 +1,36 @@
+package helpers
+
+import (
+	"fmt"
+	url2 "net/url"
+)
+
+// ConvertToHostname strips a url from everything but the hostname part
+func ConvertToHostname(url string) (string, string, error) {
+	urlWithSchema := fmt.Sprintf("x://%s", url)
+	u, err := url2.Parse(urlWithSchema)
+	if err != nil {
+		return "", "", err
+	}
+	hostName := u.Hostname()
+	port := u.Port()
+
+	return hostName, port, err
+}
+
+// NormalizeRegistry makes sure variations of DockerHubs registry
+func NormalizeRegistry(registry string) (string, error) {
+	hostName, port, err := ConvertToHostname(registry)
+	if err != nil {
+		return "", err
+	}
+
+	if hostName == "registry-1.docker.io" || hostName == "docker.io" {
+		hostName = "index.docker.io"
+	}
+
+	if port != "" {
+		return fmt.Sprintf("%s:%s", hostName, port), nil
+	}
+	return hostName, nil
+}