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.yaml

@@ -9,3 +9,7 @@
 - name: Ensure Docker daemon configuration
   ansible.builtin.import_tasks:
     file: main/03_docker_config.yaml
+
+- name: Ensure automatic cleanup of unused Docker data is set up
+  ansible.builtin.import_tasks:
+    file: main/04_docker_auto_cleanup.yaml