archive: consistently respect value of WithSkipDockerManifest

It was possible to still export the docker-compatible manifest.json
file, if a single platform image (as a standalone manifest) was
exported, even if the WithSkipDockerManifest option was explicitly set.

To resolve this, we remove all references to skipDockerManifest to,
adding it instead to the point-of-writing, simplifying the earlier logic
and making it clear exactly when this manifest file should be written.

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

images/archive/exporter.go

@@ -188,7 +188,7 @@ func Export(ctx context.Context, store content.Provider, writer io.Writer, opts
 			}
 
 			name := desc.Annotations[images.AnnotationImageName]
-			if name != "" && !eo.skipDockerManifest {
+			if name != "" {
 				mt.names = append(mt.names, name)
 			}
 		case images.MediaTypeDockerSchema2ManifestList, ocispec.MediaTypeImageIndex:
@@ -227,26 +227,24 @@ func Export(ctx context.Context, store content.Provider, writer io.Writer, opts
 					records = append(records, r...)
 				}
 
-				if !eo.skipDockerManifest {
-					if len(manifests) >= 1 {
-						if len(manifests) > 1 {
-							sort.SliceStable(manifests, func(i, j int) bool {
-								if manifests[i].Platform == nil {
-									return false
-								}
-								if manifests[j].Platform == nil {
-									return true
-								}
-								return eo.platform.Less(*manifests[i].Platform, *manifests[j].Platform)
-							})
-						}
-						d = manifests[0].Digest
-						dManifests[d] = &exportManifest{
-							manifest: manifests[0],
-						}
-					} else if eo.platform != nil {
-						return fmt.Errorf("no manifest found for platform: %w", errdefs.ErrNotFound)
+				if len(manifests) >= 1 {
+					if len(manifests) > 1 {
+						sort.SliceStable(manifests, func(i, j int) bool {
+							if manifests[i].Platform == nil {
+								return false
+							}
+							if manifests[j].Platform == nil {
+								return true
+							}
+							return eo.platform.Less(*manifests[i].Platform, *manifests[j].Platform)
+						})
+					}
+					d = manifests[0].Digest
+					dManifests[d] = &exportManifest{
+						manifest: manifests[0],
 					}
+				} else if eo.platform != nil {
+					return fmt.Errorf("no manifest found for platform: %w", errdefs.ErrNotFound)
 				}
 				resolvedIndex[desc.Digest] = d
 			}
@@ -262,7 +260,7 @@ func Export(ctx context.Context, store content.Provider, writer io.Writer, opts
 		}
 	}
 
-	if len(dManifests) > 0 {
+	if !eo.skipDockerManifest && len(dManifests) > 0 {
 		tr, err := manifestsRecord(ctx, store, dManifests)
 		if err != nil {
 			return fmt.Errorf("unable to create manifests file: %w", err)