ci: add automated release pipeline with Docker Hub push and China mirror support

- Fix Go/Node version mismatch in CI (1.21→1.25, 18→20)
- Rewrite release.yml: 3-job pipeline (frontend → binary release + Docker push)
  - Supports both tag push and manual workflow_dispatch trigger
  - Builds linux/amd64 + linux/arm64 binaries → GitHub Release tar.gz
  - Builds multi-arch Docker image → Docker Hub (awuqing/backupx)
- Dockerfile: add ARG USE_CHINA_MIRROR for China network acceleration
  (npm→npmmirror, go→goproxy.cn, apk→aliyun), add ARG VERSION injection
- Makefile: auto version from git tag, add docker/docker-cn targets
- README: add beginner-friendly China build guide and release instructions

.github/workflows/ci.yml

@@ -16,7 +16,7 @@ jobs:
       - name: Set up Go
         uses: actions/setup-go@v5
         with:
-          go-version: '1.21'
+          go-version: '1.25'
           cache-dependency-path: server/go.sum
 
       - name: Build
@@ -36,7 +36,7 @@ jobs:
       - name: Set up Node.js
         uses: actions/setup-node@v4
         with:
-          node-version: '18'
+          node-version: '20'
           cache: 'npm'
           cache-dependency-path: web/package-lock.json