`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.

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.

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.

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