Vendor in latest containers/buildah

Switch from projectatomic/buildah to containers/buildah

Signed-off-by: Daniel J Walsh <dwalsh@redhat.com>

vendor/github.com/containers/buildah/mount.go

@@ -0,0 +1,17 @@
+package buildah
+
+// Mount mounts a container's root filesystem in a location which can be
+// accessed from the host, and returns the location.
+func (b *Builder) Mount(label string) (string, error) {
+	mountpoint, err := b.store.Mount(b.ContainerID, label)
+	if err != nil {
+		return "", err
+	}
+	b.MountPoint = mountpoint
+
+	err = b.Save()
+	if err != nil {
+		return "", err
+	}
+	return mountpoint, nil
+}