fix: move extra_hosts under backend_options.docker (v3.13 schema)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.woodpecker.yml

@@ -9,6 +9,8 @@ steps:
   - name: build
     image: woodpeckerci/plugin-kaniko
     # 把 registry 域名指到宿主机内网 IP, 镜像层推送走本地不绕公网
+    backend_options:
+      docker:
         extra_hosts:
           - "registry.woni.link:172.19.152.186"
     settings: