Revert the removal of buildah dependency

We currently need it to drag in recent versions of other dependencies,
per https://github.com/containers/skopeo/issues/796 .

I'll work to update the relevant dependencies in c/image, but that will
only propagate to skopeo in the next c/image release; in the meantime,
this at least undoes the downgrades.

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

vendor/github.com/docker/distribution/blobs.go

@@ -10,6 +10,7 @@ import (
 
 	"github.com/docker/distribution/reference"
 	"github.com/opencontainers/go-digest"
+	"github.com/opencontainers/image-spec/specs-go/v1"
 )
 
 var (
@@ -66,12 +67,19 @@ type Descriptor struct {
 	Size int64 `json:"size,omitempty"`
 
 	// Digest uniquely identifies the content. A byte stream can be verified
-	// against against this digest.
+	// against this digest.
 	Digest digest.Digest `json:"digest,omitempty"`
 
 	// URLs contains the source URLs of this content.
 	URLs []string `json:"urls,omitempty"`
 
+	// Annotations contains arbitrary metadata relating to the targeted content.
+	Annotations map[string]string `json:"annotations,omitempty"`
+
+	// Platform describes the platform which the image in the manifest runs on.
+	// This should only be used when referring to a manifest.
+	Platform *v1.Platform `json:"platform,omitempty"`
+
 	// NOTE: Before adding a field here, please ensure that all
 	// other options have been exhausted. Much of the type relationships
 	// depend on the simplicity of this type.