Support hostnames and domains in PIHOLE_DNS_

This is a follow up to Start: Support hostnames and domains in PIHOLE_DNS_ by networkException · Pull Request #816 · pi-hole/docker-pi-hole · GitHub #docker

Currently the docker configuration does not allow for a hostname or domain in the pihole dns configuration. Technically DNS servers are never domains but in some cases the ip of a domain might want to be used as the DNS server. In particular a docker-compose stack would benefit from this feature:

services:
    pihole:
        image: pihole/pihole
        hostname: pihole        
        depends_on:
            - dnscrypt        
        ports:
            - 53:53/udp
        environment:
            PIHOLE_DNS_: dnscrypt

    dnscrypt:
        image: #...

By resolving the ip of "dnscrypt" on startup, pihole could now use the dnscrypt container as a DNS server.

@PromoFaux Could you check this out?