add better generate

Signed-off-by: Jess Frazelle <acidburn@microsoft.com>

vendor/github.com/genuinetools/reg/registry/digest.go

@@ -0,0 +1,34 @@
+package registry
+
+import (
+	"fmt"
+	"net/http"
+
+	"github.com/docker/distribution/manifest/schema2"
+)
+
+// Digest returns the digest for a repository and reference.
+func (r *Registry) Digest(repository, ref string) (string, error) {
+	url := r.url("/v2/%s/manifests/%s", repository, ref)
+	r.Logf("registry.manifests.get url=%s repository=%s ref=%s",
+		url, repository, ref)
+
+	req, err := http.NewRequest("GET", url, nil)
+	if err != nil {
+		return "", err
+	}
+	req.Header.Set("Accept", schema2.MediaTypeManifest)
+
+	resp, err := r.Client.Do(req)
+	if err != nil {
+		return "", err
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK && resp.StatusCode != http.StatusNotFound {
+		return "", fmt.Errorf("Got status code: %d", resp.StatusCode)
+	}
+
+	digest := resp.Header.Get("Docker-Content-Digest")
+	return digest, nil
+}