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/registry/api/errcode/handler.go

@@ -36,9 +36,5 @@ func ServeJSON(w http.ResponseWriter, err error) error {
 
 	w.WriteHeader(sc)
 
-	if err := json.NewEncoder(w).Encode(err); err != nil {
-		return err
-	}
-
-	return nil
+	return json.NewEncoder(w).Encode(err)
 }

vendor/github.com/docker/distribution/registry/api/v2/routes.go

@@ -14,15 +14,6 @@ const (
 	RouteNameCatalog         = "catalog"
 )
 
-var allEndpoints = []string{
-	RouteNameManifest,
-	RouteNameCatalog,
-	RouteNameTags,
-	RouteNameBlob,
-	RouteNameBlobUpload,
-	RouteNameBlobUploadChunk,
-}
-
 // Router builds a gorilla router with named routes for the various API
 // methods. This can be used directly by both server implementations and
 // clients.