docker(role): move automatic cleanup of unused Docker data here

Move the automatic cleanup of unused Docker data to the docker role from
the docker_compose role, so that hosts, which only use Docker (like
renovate) also have an automatic cleanup set up.
Also use a systemd timer instead of cron.

roles/docker/tasks/main/04_docker_auto_cleanup.yaml

@@ -0,0 +1,28 @@
+- name: ensure systemd service exists
+  ansible.builtin.copy:
+    src: docker-cleanup.service
+    dest: /etc/systemd/system/docker-cleanup.service
+    owner: root
+    group: root
+    mode: "0644"
+  become: true
+  notify:
+    - systemd daemon reload
+
+- name: ensure systemd timer exists
+  ansible.builtin.copy:
+    src: docker-cleanup.timer
+    dest: /etc/systemd/system/docker-cleanup.timer
+    owner: root
+    group: root
+    mode: "0644"
+  become: true
+  notify:
+    - systemd daemon reload
+
+- name: ensure systemd timer is started and enabled
+  ansible.builtin.systemd_service:
+    name: docker-cleanup.timer
+    state: started
+    enabled: true
+  become: true