Corrected the custom domain server setup

Apache's default Listen 80 and Listen 443 take those ports on every address, so Caddy could not bind the second address. The setup now gives Apache Listen lines for Magpie's own address and 127.0.0.1:8080 only. An AAAA record for domains.magpie.diy is added only with Caddy bound to that IPv6 address. On a DigitalOcean droplet, the reserved IP is the address customers point at and Caddy binds the droplet's anchor IP. A new step checks from the server that the public address reaches Caddy, which the worker's HTTPS check needs.