Pull chain: 12 CN mirrors + EVALHARNESS_DOCKER_MIRRORS override

Was 6 explicit sources (the daemon's first hop hides 12 more of its
own); added 1panel.live, dockerproxy, 163, baidubce, ustc, sjtug, nju,
tuna. With the 10s idle watchdog the full sweep costs ~2min worst
case. EVALHARNESS_DOCKER_MIRRORS replaces the whole list.

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

evalharness/sandbox/prefetch.py

@@ -23,10 +23,22 @@ _CN_MIRROR_FALLBACKS = [
     '{img}',  # daemon default (uses its own registry-mirrors config)
     'docker.m.daocloud.io/{img}',
     'docker.1ms.run/{img}',
-    'mirror.baidubce.com/{img}',
-    'docker.mirrors.sjtug.sjtu.edu.cn/{img}',
+    'docker.1panel.live/{img}',
     'hub.rat.dev/{img}',
+    'dockerproxy.com/{img}',
+    'hub-mirror.c.163.com/{img}',
+    'mirror.baidubce.com/{img}',
+    'docker.mirrors.ustc.edu.cn/{img}',
+    'docker.mirrors.sjtug.sjtu.edu.cn/{img}',
+    'docker.nju.edu.cn/{img}',
+    'docker.mirrors.tuna.tsinghua.edu.cn/{img}',
 ]
+# 用户/环境可整体覆盖（逗号分隔模板，{img} 占位）
+import os as _os
+
+_env = _os.environ.get('EVALHARNESS_DOCKER_MIRRORS', '').strip()
+if _env:
+    _CN_MIRROR_FALLBACKS = [t.strip() for t in _env.split(',') if t.strip()]
 
 
 def images_for_dataset(ds: Dataset, limit: int = 0) -> List[str]: