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/files/docker-cleanup.service

@@ -0,0 +1,8 @@
+[Unit]
+Description=cleanup unused docker data
+After=network-online.target docker.service
+Wants=network-online.target docker.service
+
+[Service]
+Type=oneshot
+ExecStart=/usr/bin/docker system prune --all --force