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.timer

@@ -0,0 +1,9 @@
+[Unit]
+Description=cleanup unused docker data every day
+
+[Timer]
+OnCalendar=daily
+RandomizedDelaySec=1h
+
+[Install]
+WantedBy=timers.target