certbot-dns-cloudflare on asustor NAS

This will configure an Asustor NAS to use a Let's Encrypt certificate without exposing it to the internet.
To achieve this, we use Certbot with the DNS-01 challenge via Cloudflare.

I'm placing my configuration in /volume1/system. Feel free to change this to whatever location you prefer.

1. Set up Cloudflare credentials.

mkdir /volume1/system/letsencrypt