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/metrics/prometheus.go

@@ -0,0 +1,13 @@
+package metrics
+
+import "github.com/docker/go-metrics"
+
+const (
+	// NamespacePrefix is the namespace of prometheus metrics
+	NamespacePrefix = "registry"
+)
+
+var (
+	// StorageNamespace is the prometheus namespace of blob/cache related operations
+	StorageNamespace = metrics.NewNamespace(NamespacePrefix, "storage", nil)
+)