CI: add build step to deploy workflow

- Clone from local Gitea instead of actions/checkout
- Build with legacy builder (DOCKER_BUILDKIT=0) for Docker mirror support
- Use npm China mirror (registry.npmmirror.com) in Dockerfile
- Combined npm install+build in single RUN step
- Removed stale proxy env vars

dashboard/Dockerfile

@@ -2,9 +2,8 @@
 FROM node:20-alpine AS frontend
 WORKDIR /build
 COPY frontend/package.json frontend/package-lock.json* ./
-RUN npm install
 COPY frontend/ ./
-RUN npm run build
+RUN npm install --registry=https://registry.npmmirror.com && npm run build
 
 # Stage 2: Python runtime
 FROM python:3.12-slim