Docker default repo + keep stderr in worker error_text

Two live-test findings in one fix:
1. DockerExecutor has no repo fallback: when a project has no
repo_url, worker leaves task['repo'] unset and the container
host.py dispatch fails instantly with 'missing --repo' because
host-config-docker.json had no repo field either. Add the host
test-repo as the config default; projects with repo_url still
override it via worker's explicit --repo.
2. Worker discarded stderr from the final error_text, so every
container/host crash surfaced only as the generic 'host
execution failed' and was undebuggable from the task record.
When stdout is empty, error_text now keeps the stderr tail
(last 1000 chars, where the real error line lives).

cloud/worker.py

@@ -905,6 +905,13 @@ class Worker:
             parsed = None
         if isinstance(parsed, dict) and parsed.get("text"):
             return str(parsed["text"])
+        # host.py 崩溃时 stdout 常为空、真正的报错在 stderr（例如
+        # “缺少 --repo”、Read-only file system）；只用 stdout 时
+        # 最终错误会被写成通用的 host execution failed，排障无从
+        # 下手，所以 stdout 为空时取 stderr 尾部（实际报错行在尾）。
+        stderr = (result or {}).get("stderr", "") or ""
+        if not stdout.strip() and stderr.strip():
+            return stderr.strip()[-1000:]
         if token is not None:
             return "host_status=%s" % token
         return stdout.strip() or "host execution failed"
@@ -1124,7 +1131,10 @@ class Worker:
                             fallback_model, second_error))
             else:
                 if error is None:
-                    error = stdout.strip() or "host execution failed"
+                    stderr = (result or {}).get("stderr", "") or ""
+                    error = stdout.strip() or \
+                        (stderr.strip()[-1000:] if stderr.strip() else "") \
+                        or "host execution failed"
                 if retry_count:
                     error = "[retries=%d] %s" % (retry_count, error)
                 # 未配 fallback_models 时的老行为：只记事件、不重试。