Publish Alpine musl distroless Postfix as the sole latest image

build/runtime-root.py

@@ -0,0 +1,76 @@
+#!/usr/bin/env python3
+"""Assemble an Alpine-derived, shell-free runtime from an explicit ELF allowlist."""
+import os
+from pathlib import Path
+import shutil
+import subprocess
+import sys
+
+root = Path(sys.argv[1])
+root.mkdir(parents=True, exist_ok=True)
+
+copied = set()
+
+def copy(source):
+    source = Path(os.path.normpath(source))
+    if source in copied:
+        return
+    copied.add(source)
+    target = root / str(source).lstrip('/')
+    target.parent.mkdir(parents=True, exist_ok=True)
+    if source.is_symlink():
+        link = os.readlink(source)
+        if not target.is_symlink():
+            target.symlink_to(link)
+        # Preserve every hop, not just the final file in a multi-link chain.
+        copy(link if os.path.isabs(link) else source.parent / link)
+    elif source.is_dir():
+        target.mkdir(exist_ok=True)
+        for child in source.iterdir():
+            copy(child)
+    else:
+        shutil.copy2(source, target)
+
+for executable in sys.argv[2:]:
+    result = subprocess.run(['lddtreepax', '-l', executable], check=True, text=True, capture_output=True)
+    paths = result.stdout.splitlines()
+    if not paths or any(not p.startswith('/') or not Path(p).exists() for p in paths):
+        raise SystemExit(f'Unresolved runtime dependency for {executable}: {result.stdout}')
+    copy(executable)
+    for path in paths:
+        copy(path)
+
+# musl uses this file for nonstandard library directories (for example Lua).
+for search_path in Path('/etc').glob('ld-musl-*.path'):
+    copy(search_path)
+
+# Postfix loads database maps and SASL mechanisms dynamically.
+for folder in ['/usr/lib/postfix', '/usr/lib/sasl2']:
+    for plugin in Path(folder).glob('*.so*'):
+        result = subprocess.run(['lddtreepax', '-l', str(plugin)], check=True,
+                                text=True, capture_output=True,
+                                env={**os.environ, 'LD_LIBRARY_PATH': '/lib:/usr/lib:/usr/lib/postfix'})
+        for dependency in result.stdout.splitlines():
+            if not dependency.startswith('/') or not Path(dependency).exists():
+                raise SystemExit(f'Unresolved plugin dependency: {dependency}')
+            copy(dependency)
+        copy(plugin)
+for data in ['/etc/postfix', '/etc/passwd', '/etc/group', '/etc/aliases',
+             '/etc/aliases.lmdb', '/etc/ssl/certs/ca-certificates.crt', '/etc/ssl/cert.pem',
+             '/etc/ssl/openssl.cnf', '/usr/lib/ossl-modules', '/usr/share/zoneinfo',
+             '/usr/share/icu', '/usr/lib/icu', '/etc/services', '/etc/protocols', '/etc/alpine-release', '/etc/os-release']:
+    if Path(data).exists():
+        copy(data)
+for directory in ['tmp', 'root', 'dev', 'var/mail', 'var/lib/postfix', 'var/spool/postfix']:
+    (root / directory).mkdir(parents=True, exist_ok=True)
+os.chmod(root / 'tmp', 0o1777)
+# Preserve the Postfix command group and setgid bits for queue submission.
+for command in ['postdrop', 'postqueue', 'postlog']:
+    source = Path('/usr/sbin') / command
+    target = root / str(source).lstrip('/')
+    stat = source.stat()
+    os.chown(target, stat.st_uid, stat.st_gid)
+    os.chmod(target, stat.st_mode & 0o7777)
+for forbidden in ['bin/sh', 'bin/bash', 'bin/busybox', 'sbin/apk', 'usr/bin/apk',
+                  'usr/bin/python3', 'usr/bin/go', 'usr/sbin/postfix']:
+    assert not (root / forbidden).exists(), f'Unexpected runtime tool: {forbidden}'