fix: make backend Docker builds reproducible

backend/tests/architecture-tests.js

@@ -91,6 +91,11 @@ test('Docker 构建上下文排除依赖、环境配置和运行时数据', () =
   for (const requiredEntry of ['node_modules', '.env', 'data/*', 'storage/*']) {
     assert.ok(dockerIgnore.split(/\r?\n/).includes(requiredEntry), requiredEntry);
   }
+
+  const dockerfile = fs.readFileSync(path.join(projectRoot, 'backend', 'Dockerfile'), 'utf8');
+  assert.ok(dockerfile.includes('AS dependencies'));
+  assert.ok(dockerfile.includes('AS runtime'));
+  assert.ok(dockerfile.includes('npm_config_nodedir=/usr/local'));
 });
 
 test('install.sh 生成的 Nginx server 块花括号平衡', () => {