Add remote.Digest() to upload a manifest via digest (#980)

Signed-off-by: Avi Deitcher <avi@deitcher.net>

pkg/v1/remote/write.go

@@ -849,27 +849,42 @@ func WriteLayer(repo name.Repository, layer v1.Layer, options ...Option) (rerr e
 // should ensure that all blobs or manifests that are referenced by t exist
 // in the target registry.
 func Tag(tag name.Tag, t Taggable, options ...Option) error {
-	o, err := makeOptions(tag.Context(), options...)
+	return Put(tag, t, options...)
+}
+
+// Put adds a manifest from the given Taggable via PUT /v1/.../manifest/<ref>
+//
+// Notable implementations of Taggable are v1.Image, v1.ImageIndex, and
+// remote.Descriptor.
+//
+// If t implements MediaType, we will use that for the Content-Type, otherwise
+// we will default to types.DockerManifestSchema2.
+//
+// Put does not attempt to write anything other than the manifest, so callers
+// should ensure that all blobs or manifests that are referenced by t exist
+// in the target registry.
+func Put(ref name.Reference, t Taggable, options ...Option) error {
+	o, err := makeOptions(ref.Context(), options...)
 	if err != nil {
 		return err
 	}
-	scopes := []string{tag.Scope(transport.PushScope)}
+	scopes := []string{ref.Scope(transport.PushScope)}
 
 	// TODO: This *always* does a token exchange. For some registries,
 	// that's pretty slow. Some ideas;
 	// * Tag could take a list of tags.
 	// * Allow callers to pass in a transport.Transport, typecheck
 	//   it to allow them to reuse the transport across multiple calls.
 	// * WithTag option to do multiple manifest PUTs in commitManifest.
-	tr, err := transport.NewWithContext(o.context, tag.Context().Registry, o.auth, o.transport, scopes)
+	tr, err := transport.NewWithContext(o.context, ref.Context().Registry, o.auth, o.transport, scopes)
 	if err != nil {
 		return err
 	}
 	w := writer{
-		repo:    tag.Context(),
+		repo:    ref.Context(),
 		client:  &http.Client{Transport: tr},
 		context: o.context,
 	}
 
-	return w.commitManifest(t, tag)
+	return w.commitManifest(t, ref)
 }