fix variable quoting issue

Change-Id: Icb4770d8a36b65366eb30ff3025d182447e66e22
diff --git a/ansible/roles/docker/tasks/centos.yml b/ansible/roles/docker/tasks/centos.yml
index a8910d4..6bd3d7e 100644
--- a/ansible/roles/docker/tasks/centos.yml
+++ b/ansible/roles/docker/tasks/centos.yml
@@ -2,7 +2,7 @@
   copy:
     src: "{{ item.src }}"
     dest: "{{ item.dest }}"
-  with_items: centos_files
+  with_items: "{{ centos_files }}"
   tags: [docker]
 
 - name: CentOS package is installed