Phase 5: full outbound Postfix relay (465/587 SASL+TLS, sender binding, milters)

Generate the relay config from the environment at container start
(build/postfix-config.sh, run from entrypoint.sh):

- smtps 465 (implicit TLS) primary + optional submission 587 (STARTTLS),
  chroot=n so smtpd reaches the sasldb2/sender map under /data.
- Cyrus SASL against the panel-maintained sasldb2; realm left implicit so the
  authenticated name equals the bare login in smtpd_sender_login_maps.
- reject_sender_login_mismatch + relay/recipient restrictions with no
  permit_mynetworks: credentials-only, open relay impossible (spec 5, 5.1).
- TLS cert/key from TLS_CERT_FILE/TLS_KEY_FILE; daily postfix reload picks up
  renewed certs (postfix-cert-reload.sh under supervisord, spec 5.2).
- anvil level-1 rate limit from env (spec 5 p.5).
- Milter chain with per-milter action: OpenDKIM strict (tempfail), journal
  fail-open (accept) so monitoring never blocks the relay (spec 7.3).

Two integration fixes found on the server:
- postconf -F '*/*/chroot=n': Debian's chrooted delivery agent can't read
  /etc/resolv.conf, so MX lookups failed and mail never left.
- entrypoint sets /run/opendkim and /run/selfpost to group selfpost + setgid,
  and the journal stub chmods its socket 0660, so postfix can connect to both
  milter sockets (strict OpenDKIM was milter-rejecting all mail otherwise).

Verified on selfpost.mixfed.ru: gofmt/vet/test green, image builds; container
e2e — 465 auth+send DKIM-signed (d=domain,s=selfpost), 587 STARTTLS auth,
cross-domain sender 553, list-mode per-address binding, unauth relay 554,
real outbound delivery reaching the recipient MX over TLS.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

build/postfix-cert-reload.sh

@@ -0,0 +1,25 @@
+#!/bin/sh
+# Periodic `postfix reload` so refreshed TLS certificates are picked up (spec
+# 5.2 p.4). The reverse-proxy renews the PEM files in the read-only mount every
+# few months; Postfix only re-reads them on reload. A simple daily reload is
+# more than enough (a day of staleness is harmless) and far simpler than an
+# inotify watcher — the spec explicitly prefers this.
+#
+# Runs under supervisord as root, so it can reload Postfix directly. It sleeps
+# first, then reloads in a loop: no reload at container start (the wrapper is
+# still bringing Postfix up then) and none until at least one interval has
+# passed. A reload is harmless when nothing changed.
+set -eu
+
+INTERVAL="${TLS_RELOAD_INTERVAL_SECONDS:-86400}"
+
+while true; do
+	sleep "${INTERVAL}"
+	if postfix reload; then
+		echo "cert-reload: postfix reloaded (periodic TLS refresh)"
+	else
+		# Never exit non-zero: a transient reload failure must not trip the
+		# crashexit listener and take the container down. Log and retry next cycle.
+		echo "cert-reload: postfix reload failed, will retry after ${INTERVAL}s" >&2
+	fi
+done