Use defined syntax and make sure to create directories

plays/lib/volumes.yml

@@ -1,6 +1,17 @@
 ---
+- name: Create volume mount directories
+  become: true
+  file:
+    path: "{{ item }}"
+    state: directory
+    owner: dokku
+    group: dokku
+  with_items: "{{ mounts }}"
+  when: mounts is defined
+
 - name: Attach volume mounts
   dokku_storage:
     app: "{{ app }}"
     mounts: "{{ item }}"
-  with_items: "{{ lookup('vars', 'mounts', default=[]) }}"
+  with_items: "{{ mounts }}"
+  when: mounts is defined