imagetools: process com.docker.reference.* annotations

To give us the option later down the road of producing recommended OCI
names in BuildKit (using com instead of vnd, woops), we need to update
Buildx to be able to process both.

Ideally, if a Buildx/BuildKit release hadn't been made we could just
switch over, but since we have, we'd need to support both (at least for
a while, eventually we could consider deprecating+removing the vnd
variant).

Signed-off-by: Justin Chadwell <me@jedevc.com>

util/imagetools/loader.go

@@ -21,8 +21,11 @@ import (
 	"golang.org/x/sync/errgroup"
 )
 
-const (
-	annotationReference = "vnd.docker.reference.digest"
+var (
+	annotationReferences = []string{
+		"com.docker.reference.digest",
+		"vnd.docker.reference.digest", // TODO: deprecate/remove after migration to new annotation
+	}
 )
 
 type contentCache interface {
@@ -167,8 +170,13 @@ func (l *loader) fetch(ctx context.Context, fetcher remotes.Fetcher, desc ocispe
 		}
 		r.mu.Unlock()
 
-		ref, ok := desc.Annotations[annotationReference]
-		if ok {
+		found := false
+		for _, annotationReference := range annotationReferences {
+			ref, ok := desc.Annotations[annotationReference]
+			if !ok {
+				continue
+			}
+
 			refdgst, err := digest.Parse(ref)
 			if err != nil {
 				return err
@@ -176,7 +184,10 @@ func (l *loader) fetch(ctx context.Context, fetcher remotes.Fetcher, desc ocispe
 			r.mu.Lock()
 			r.refs[refdgst] = append(r.refs[refdgst], desc.Digest)
 			r.mu.Unlock()
-		} else {
+			found = true
+			break
+		}
+		if !found {
 			p := desc.Platform
 			if p == nil {
 				p, err = l.readPlatformFromConfig(ctx, fetcher, mfst.Config)