vendor: update buildkit to master@31c870e82a48

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

vendor/github.com/opencontainers/image-spec/specs-go/v1/manifest.go

@@ -23,6 +23,9 @@ type Manifest struct {
 	// MediaType specifies the type of this document data structure e.g. `application/vnd.oci.image.manifest.v1+json`
 	MediaType string `json:"mediaType,omitempty"`
 
+	// ArtifactType specifies the IANA media type of artifact when the manifest is used for an artifact.
+	ArtifactType string `json:"artifactType,omitempty"`
+
 	// Config references a configuration object for a container, by digest.
 	// The referenced configuration object is a JSON blob that the runtime uses to set up the container.
 	Config Descriptor `json:"config"`
@@ -36,3 +39,11 @@ type Manifest struct {
 	// Annotations contains arbitrary metadata for the image manifest.
 	Annotations map[string]string `json:"annotations,omitempty"`
 }
+
+// ScratchDescriptor is the descriptor of a blob with content of `{}`.
+var ScratchDescriptor = Descriptor{
+	MediaType: MediaTypeScratch,
+	Digest:    `sha256:44136fa355b3678a1146ad16f7e8649e94fb4fc21fe77e8310c060f61caaff8a`,
+	Size:      2,
+	Data:      []byte(`{}`),
+}