update docker task to not fail when container volumes aren't needed

roles/docker/tasks/main.yml

@@ -85,6 +85,8 @@
 - name: Generate list of persistent container directories
   set_fact:
     persistent_container_dirs: "{{ containers | map(attribute='volumes') | flatten | map('regex_replace', ':.*' ) | list }}"
+  when:
+    - containers.volumes is defined
   changed_when: false
 
 - name: Create persistent container directories
@@ -93,6 +95,8 @@
     - "{{ persistent_container_dirs }}"
   loop_control:
     label: "{{ item }}"
+  when:
+    - persistent_container_dirs is defined
 
 - name: Copy systemd service file
   template: