bump up rootlesskit (fix CentOS failure)

Changes:
https://github.com/rootless-containers/rootlesskit/compare/7905ee34b3d9d1f27fe2ffa3c5fd3d01bb3644dd...7bbbc48a6f906633a9b12783b957f4c3aa037d33

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>

hack/dockerfile/install/rootlesskit.installer

@@ -1,7 +1,7 @@
 #!/bin/sh
 
-# v0.3.0-alpha.1
-ROOTLESSKIT_COMMIT=7905ee34b3d9d1f27fe2ffa3c5fd3d01bb3644dd
+# v0.3.0-alpha.2
+ROOTLESSKIT_COMMIT=7bbbc48a6f906633a9b12783b957f4c3aa037d33
 
 install_rootlesskit() {
 	case "$1" in