go.mod: bump up github.com/opencontainers/image-spec to v1.1.0-rc5

Release note: https://github.com/opencontainers/image-spec/releases/tag/v1.1.0-rc5

Signed-off-by: Kohei Tokunaga <ktokunaga.mail@gmail.com>

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

@@ -21,7 +21,7 @@ import digest "github.com/opencontainers/go-digest"
 // when marshalled to JSON.
 type Descriptor struct {
 	// MediaType is the media type of the object this schema refers to.
-	MediaType string `json:"mediaType,omitempty"`
+	MediaType string `json:"mediaType"`
 
 	// Digest is the digest of the targeted content.
 	Digest digest.Digest `json:"digest"`

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

@@ -15,10 +15,14 @@
 package v1
 
 const (
-	// ImageLayoutFile is the file name of oci image layout file
+	// ImageLayoutFile is the file name containing ImageLayout in an OCI Image Layout
 	ImageLayoutFile = "oci-layout"
 	// ImageLayoutVersion is the version of ImageLayout
 	ImageLayoutVersion = "1.0.0"
+	// ImageIndexFile is the file name of the entry point for references and descriptors in an OCI Image Layout
+	ImageIndexFile = "index.json"
+	// ImageBlobsDir is the directory name containing content addressable blobs in an OCI Image Layout
+	ImageBlobsDir = "blobs"
 )
 
 // ImageLayout is the structure in the "oci-layout" file, found in the root

vendor/github.com/opencontainers/image-spec/specs-go/version.go

@@ -25,7 +25,7 @@ const (
 	VersionPatch = 0
 
 	// VersionDev indicates development branch. Releases will be empty string.
-	VersionDev = "-rc.4"
+	VersionDev = "-rc.5"
 )
 
 // Version is the specification version that the package types support.