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/handlers/main.yaml

@@ -0,0 +1,4 @@
+- name: systemd daemon reload
+  ansible.builtin.systemd_service:
+    daemon_reload: true
+  become: true