Feature request: support NextDNS

[mcriley821]

What's the feature 🧐

Support NextDNS as a DNS resolver for DNS-over-TLS or DNS-over-HTTPS, e.g.

DNS_UPSTREAM_RESOLVER=nextdns
DNS_NEXTDNS_ID=......

At the very least, support configuration for DNS-over-TLS / DNS-over-HTTPS using environment, e.g.

DNS_CUSTOM_TLS_RESOLVER=******.dns.nextdns.io
DNS_CUSTOM_HTTPS_RESOLVER=https://******.dns.nextdns.io

Extra information and references

I use DNS_KEEP_NAMESERVER (which points at my NextDNS), but my understanding is that this is unencrypted.

DNS_ADDRESSES looks to be unencrypted as well

[github-actions]

@qdm12 is more or less the only maintainer of this project and works on it in his free time.
Please:

• do not ask for updates, be patient
• 👍 the issue to show your support instead of commenting
  @qdm12 usually checks issues at least once a week, if this is a new urgent bug,
  revert to an older tagged container image