Lets downstreams add per-service env vars (e.g. HOST for Caddy's
Dynu DNS-01 challenge) without breaking the shared DOMAIN alias.

Comments said 'Replace example.com everywhere' which became stale once
$DOMAIN drives haproxy.cfg + Caddyfile + docker-compose. Reword so the
user's mental model is: pick a domain, generate the secret with it,
put it in .env once.

Closes #501.

Before: the SNI hostname had to be edited in haproxy.cfg, plus
DOMAIN had to be set in .env for Caddy.  Two places to keep in sync.

After: DOMAIN is the single source — set it once in .env (or export),
docker-compose forwards it into both haproxy and caddy containers,
and HAProxy interpolates ${DOMAIN} into the SNI ACL at startup.

mtg-config.toml's secret still embeds the domain in its bytes
(generated once with `mtg generate-secret <domain>`), so that one
remains a one-time edit at install — the README is updated to reflect
this.

HAProxy has supported environment-variable substitution in config
strings since 1.6.

Might be needed for local chained proxy setup.

Fix SELinux-related permission denied error for containerized apps
reading configs exposed via volumes.
Also make it possible to use port 80 in the fronted.

Without this, mtg and Caddy see HAProxy's container IP for every
connection, which breaks meaningful logging, abuse handling, and any
IP-based blocklist logic.  HAProxy sends a PROXY protocol v2 header on
its TCP backends; mtg enables proxy-protocol-listener, and Caddy wraps
:8443 with a proxy_protocol listener before tls.

The :80 path (ACME HTTP-01 passthrough) is unchanged — client IP there
is not useful and HAProxy's http mode already adds X-Forwarded-For if
anyone wants it.

Requested in https://github.com/9seconds/mtg/pull/462 review.

Add an ACL that routes /.well-known/acme-challenge/ requests on :80
to Caddy instead of redirecting to HTTPS, so Let's Encrypt certificate
issuance works out of the box.

Also simplify Caddyfile to use Caddy's http_port/https_port directives.

Turnkey deployment: HAProxy on :443 peeks at the TLS SNI and routes
Telegram clients to mtg while forwarding everything else (including DPI
probes) to a real Caddy web server with automatic HTTPS.

This is the setup recommended in BEST_PRACTICES.md, packaged so that
operators can clone and run it with minimal configuration.

Refs: #458