Phase 1: Docker image, supervisord, three-process cold start

Single bookworm-slim image running opendkim + panel + postfix under
supervisord with enforced start ordering (spec 4):

- build/Dockerfile: multi-stage static Go build; runtime installs postfix,
  opendkim, cyrus-sasl, supervisor, logrotate; unprivileged panel user (7.6.8).
- build/supervisord.conf: priority ordering opendkim -> panel -> postfix;
  crashexit event listener terminates the container on any FATAL process.
- build/postfix-wrapper.sh: waits for both milter sockets (test -S, 30s
  timeout) before `postfix start-fg`, exits non-zero on timeout.
- panel: HTTP :8080 stub + /healthz, journal-milter socket stub (so the
  wrapper's readiness probe passes), log-tailer stub; SIGTERM graceful stop.

Verified on the dev server: image builds, three processes live, panel serves
the stub, wrapper waits for sockets, and an unrecoverable panel failure brings
the container down cleanly.

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

cmd/panel/journalstub.go

@@ -0,0 +1,48 @@
+package main
+
+import (
+	"context"
+	"errors"
+	"log"
+	"net"
+	"os"
+	"path/filepath"
+)
+
+// serveJournalStub opens the journal-milter Unix socket so the Postfix start
+// wrapper's readiness probe (test -S) succeeds and the cold-start ordering
+// (spec 4) can be exercised end to end. The real milter protocol handler is
+// implemented in Phase 6; here connections are simply accepted and closed.
+func serveJournalStub(ctx context.Context, socketPath string) error {
+	if err := os.MkdirAll(filepath.Dir(socketPath), 0o755); err != nil {
+		return err
+	}
+	// Clear a stale socket left behind by an unclean shutdown, otherwise the
+	// listen below fails with "address already in use".
+	if err := os.Remove(socketPath); err != nil && !errors.Is(err, os.ErrNotExist) {
+		return err
+	}
+
+	ln, err := net.Listen("unix", socketPath)
+	if err != nil {
+		return err
+	}
+
+	// Closing the listener unblocks Accept and unlinks the socket file.
+	go func() {
+		<-ctx.Done()
+		_ = ln.Close()
+	}()
+
+	log.Printf("journal-milter stub listening on %s", socketPath)
+	for {
+		conn, err := ln.Accept()
+		if err != nil {
+			if ctx.Err() != nil {
+				return nil // expected during shutdown
+			}
+			return err
+		}
+		_ = conn.Close()
+	}
+}