Switch to one-line `bind :80,[::]:80` and `bind :443,[::]:443` per
review feedback in #522.  The v6only flag was self-documentation, not
load-bearing: with SO_REUSEADDR (HAProxy's default) and bindv6only=0
the kernel routes v4 packets to the more-specific AF_INET socket
regardless.  Comment trimmed to match — the v6only paragraph is gone
because v6only itself is gone.

The shorter form also scales more cleanly when adding ports later,
e.g. `bind :8080,[::]:8080` on a new line.

- Caddy allow: 127.0.0.0/8 → 127.0.0.1/32 (only loopback peer is HAProxy).
- haproxy.cfg: rewrite v6only comment to describe what it actually does
  (suppresses v4-mapped accept, preventing conflict with the v4 bind),
  not the symptom.
- docker-compose.yml: trim the 8-line haproxy comment to 3 lines and
  defer the rationale to README.  Add one-line note explaining why web
  uses host port 8080 (HAProxy owns :80).
- README: condense the "Why network_mode: host" subsection.  Spell out
  trade-offs as a list: own-the-host-ports, Linux-only (Docker Desktop
  doesn't make this layout reachable), userns-remap incompatibility.
  Note that mtg-config.toml stays as-is because mtg/web remain on the
  compose bridge.

Bridge ingress (Docker's docker-proxy userland forwarder, Podman's
slirp4netns/pasta) rewrites the source IP of inbound connections on a
published port to the bridge gateway address.  HAProxy then stamps that
gateway address into the PROXY v2 header it forwards to mtg and Caddy,
so neither backend ever sees a real client IP.

Move HAProxy into the host netns (network_mode: host) so it binds
:443/:80 directly with no NAT in the path.  mtg and Caddy stay on the
compose bridge and are published on 127.0.0.1 only; HAProxy reaches
them via host loopback and PROXY v2 carries the real client IP (v4 or
v6) end-to-end.

Also accept IPv6 clients explicitly on the HAProxy frontends — `bind
*:443` is IPv4-only and missed v6 clients on hosts where the previous
example happened to "work" only because of dual-stack quirks.

Add 127.0.0.0/8 to Caddy's PROXY allow-list to cover the new loopback
hop from HAProxy.  README gains a short subsection explaining the
host-mode choice and its trade-off (HAProxy occupies host :443/:80).

Diagnosed and tested by @bam80 on Fedora + Docker 29.  Fixes #498.

`MTG_SECRET=<placeholder> envsubst < ...` was shell-broken on literal
copy-paste — bash parses `<placeholder>` as redirection from a
non-existent file. Two-line `export MTG_SECRET=...` + plain envsubst
form removes the ambiguity. Applies to README, docker-compose.yml,
and the .example header.

Track `mtg-config.toml.example` with `secret = "${MTG_SECRET}"`; the
rendered `mtg-config.toml` and local `.env` are gitignored, so the
secret never lands in a tracked file.

Quick start switches from "paste the secret into mtg-config.toml" to
either `envsubst < mtg-config.toml.example > mtg-config.toml` or
`cp` + hand-edit `${MTG_SECRET}` for users without envsubst.

After #502 made DOMAIN env-driven, the secret was the last hand-edit
of a tracked file in the example. Follow-up to #506.

OpenWrt firewall zones are bound to interface names. With bare podman
you can pin the static podman0 bridge into a zone, but podman-compose
creates a project-scoped network and netavark spawns a fresh bridge
(podman1, podman2, ...) per project — with no firewall rules — so
containers lose outbound access.

Mark the default network as external/name=podman to attach to the
router-managed podman0 instead.

Background: #513.

Fix #512

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

mtg now also sends PROXY v2 on the fronting dial (introduced in the
previous commit via [domain-fronting].proxy-protocol = true), so the
"Real client IPs" section's sync list must include that fourth piece.
Without it, an operator who disables Caddy's PROXY listener wrapper
without also flipping [domain-fronting].proxy-protocol will leave mtg
sending an unparsed PROXY v2 prefix to Caddy on every fronted probe.

When the secret's domain resolves back to this server (the SNI-router
default), mtg's fallback fronting dial lands on HAProxy, the SNI
matches the secret, HAProxy routes the connection back to mtg -> loop.

Set [domain-fronting].host = "web" in mtg-config.toml so mtg dials
Caddy directly via compose-network DNS, bypassing HAProxy.  Requires
mtg >= 2.4 (#480 added hostname acceptance for the fronting target).

README gains a "Fronting loop" section explaining the cause.

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