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_compose/tasks/main.yaml

@@ -108,11 +108,3 @@
     minute: "0"
     hour: "5"
     job: "cd /ansible_docker_compose; docker compose pull && docker compose up -d"
-
-- name: ensure automatic cleanup cron job is present
-  become: true
-  ansible.builtin.cron:
-    name: 'ansible docker compose auto cleanup'
-    minute: "23"
-    hour: "4"
-    job: "docker system prune -a -f"