update docker role & add separate play

roles/docker/tasks/users.yml

@@ -0,0 +1,12 @@
+---
+- name: Ensure docker group exists
+  ansible.builtin.group:
+    name: docker
+    state: present
+
+- name: Add users to the docker group
+  ansible.builtin.user:
+    name: "{{ item }}"
+    groups: docker
+    append: true
+  loop: "{{ docker_users }}"