caddy: add caddy-dns/rfc2136 + test-rfc2136 site -- self-hosted ACME flow

Wires Caddy as the ACME client side of our new self-hosted DNS-01
flow. Proves the design end-to-end: caddy-dns/rfc2136 -> our
CoreDNS rfc2136 plugin -> zone file write -> git auto-commit -> HE
AXFR -> LE validates -> cert issued.

Changes:
- caddy/Dockerfile: --with github.com/caddy-dns/rfc2136 added
  alongside the existing caddy-dns/vultr.
- caddy/Caddyfile: new test-rfc2136.supported.systems site that uses
  the new provider. server coredns:53 (docker internal), key from
  env, propagation_delay 60s + timeout 600s to accommodate HE pull.
- docker-compose.yml: ACME_TSIG_SECRET passed to the caddy container
  (the same secret CoreDNS verifies on the other side of the loop).

First cert issued in production: 2026-05-21 ~13:23 UTC. ~5.5 min
end-to-end from Caddy starting to cert in hand. Documented in
session notes; the cert sits unused in caddy-data/ until/unless
something publishes ports 80/443 for that hostname.

docker-compose.yml

@@ -11,6 +11,9 @@ services:
     environment:
       - CADDY_HOSTNAME=${CADDY_HOSTNAME}
       - ACME_EMAIL=${ACME_EMAIL}
+      # Used by caddy-dns/rfc2136 for the test-rfc2136 site -- same
+      # secret CoreDNS's rfc2136 plugin verifies on the other side.
+      - ACME_TSIG_SECRET=${ACME_TSIG_SECRET}
       # Optional: only required for Caddy's DNS-01 cert renewal via Vultr's
       # API. Cert is valid ~90 days; this env var only matters within the
       # final 30d renewal window. Empty default keeps `docker compose up`