docker_compose(role): add support for deploying optional .env file

This is needed for situations, where one wants to use a vendor-provided
compose file and configure it using environment variables.
Like for example:
https://github.com/zammad/zammad-docker-compose

roles/docker_compose/tasks/main.yaml

@@ -17,6 +17,17 @@
   become: true
   notify: docker compose down
 
+- name: deploy the .env file
+  ansible.builtin.copy:
+    content: "{{ docker_compose__env_file_content }}"
+    dest: /ansible_docker_compose/.env
+    mode: "0644"
+    owner: root
+    group: root
+  become: true
+  when: docker_compose__env_file_content is defined
+  notify: docker compose down
+
 - name: make sure the `/ansible_docker_compose/configs` directory exists
   ansible.builtin.file:
     path: /ansible_docker_compose/configs