fix subject for oci artifact of attestation manifest

Signed-off-by: CrazyMax <[email protected]>

Author: crazy-max

client/client_test.go

@@ -9613,7 +9613,9 @@ func testExportAttestations(t *testing.T, sb integration.Sandbox, ociArtifact bo
 			if ociArtifact {
 				subject := att.Manifest.Subject
 				require.NotNil(t, subject)
-				require.Equal(t, bases[i].Desc, *subject)
+				require.Equal(t, bases[i].Desc.MediaType, subject.MediaType)
+				require.Equal(t, bases[i].Desc.Digest, subject.Digest)
+				require.Equal(t, bases[i].Desc.Size, subject.Size)
 				require.Equal(t, "application/vnd.docker.attestation.manifest.v1+json", att.Manifest.ArtifactType)
 				require.Equal(t, ocispecs.DescriptorEmptyJSON, att.Manifest.Config)
 			} else {

exporter/containerimage/writer.go

@@ -617,7 +617,11 @@ func (ic *ImageWriter) commitAttestationsManifest(ctx context.Context, opts *Ima
 
 	if ociArtifact {
 		mfst.ArtifactType = attestationManifestArtifactType
-		mfst.Subject = &target
+		mfst.Subject = &ocispecs.Descriptor{
+			Digest:    target.Digest,
+			Size:      target.Size,
+			MediaType: target.MediaType,
+		}
 	}
 
 	labels := map[string]string{