By default, docker doesn't clean up old images that are used to build the firmware and can easily fill up the storage space on the server. To manage this, a script is run periodically to prune old images. This script is stored at `/etc/cron.daily/docker-prune` and is executed automatically every day by the corn daemon. The script logs all its output to `/var/lib/docker/prune.log`
```bash
#!/bin/bash
log=/var/lib/docker/prune.log
date +'=== %Y.%m.%d %H:%M ==='>>$log
#remove all dangling docker images that are older than 15 days
docker system prune -f--filter"until=$((15*24))h">>$log
