spec-go/: scratch descriptor for config and layer

to facilitate use by tools that import this reference code to identify
this specific blob when working with an empty/scratch config blob and
layer blob. This will utilize the same blob for both

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

specs-go/v1/manifest.go

@@ -36,3 +36,16 @@ type Manifest struct {
 	// Annotations contains arbitrary metadata for the image manifest.
 	Annotations map[string]string `json:"annotations,omitempty"`
 }
+
+// ScratchDigestSHA256 is the digest of a blob with content of `{}` (size of 2)
+const ScratchDigestSHA256 = `sha256:44136fa355b3678a1146ad16f7e8649e94fb4fc21fe77e8310c060f61caaff8a`
+
+// ScratchDescriptor is a SCRATCH descriptor for Manifest's `config` when that manifest does not have a config to provide.
+func ScratchDescriptor() Descriptor {
+	return Descriptor{
+		MediaType: MediaTypeImageScratch,
+		Digest:    ScratchDigestSHA256,
+		Size:      2,
+		Data:      []byte(`{}`),
+	}
+}

specs-go/v1/mediatype.go

@@ -70,6 +70,9 @@ const (
 	// MediaTypeImageConfig specifies the media type for the image configuration.
 	MediaTypeImageConfig = "application/vnd.oci.image.config.v1+json"
 
+	// MediaTypeImageScratch specifies the media type for a SCRATCH blob.
+	MediaTypeImageScratch = "application/vnd.oci.example+json"
+
 	// MediaTypeArtifactManifest specifies the media type for a content descriptor.
 	MediaTypeArtifactManifest = "application/vnd.oci.artifact.manifest.v1+json"
 )