fix(deploy): keep BuildKit cache and use Huawei mirrors for API images

Gitea run 2828 validated, then publish hit the 60-minute deadline while
Aliyun apt/pip crawled at about 22 kB/s after a 72h cache prune on a
runner with 677 GiB free. Skip BuildKit prune when disk is above the
minimum, split API apt/pip layers, and point apt/pypi at Huawei Cloud.

tests/test_railway_deployment.py

@@ -15,6 +15,11 @@ def test_railway_services_use_the_product_frontend_and_dynamic_ports() -> None:
 
     assert "FROM swr.cn-north-4.myhuaweicloud.com/ddn-k8s/docker.io/library/python:3.12-slim" in api
     assert "daocloud" not in api.lower()
+    assert "PIP_INDEX_URL=https://mirrors.huaweicloud.com/repository/pypi/simple/" in api
+    assert "mirrors.aliyun.com" not in api
+    assert api.index("apt-get install -y --no-install-recommends build-essential nodejs") < api.index(
+        "python -m pip install -r requirements.txt"
+    )
     assert "COPY SKILL.md mcp_server.py" in api
     assert "--host 0.0.0.0" in api and "${PORT:-5200}" in api
     assert "http.server" not in api