Add become: true and fix loop var name

tasks/docker-users.yml

@@ -1,9 +1,12 @@
 ---
 - name: Ensure docker users are added to the docker group.
+  become: true
   user:
-    name: "{{ item }}"
+    name: "{{ muser }}"
     groups: docker
     append: true
+  loop_control:
+    loop_var:  muser
   with_items: "{{ docker_users }}"
 
 - name: Reset ssh connection to apply user changes.