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

Makefile

@@ -1,22 +1,25 @@
-.PHONY: build dev test clean
+.PHONY: build dev test clean docker docker-cn
 
-# 一次性构建前后端
+# 自动获取版本号（从 git tag 或 commit hash）
+VERSION ?= $(shell git describe --tags --always --dirty 2>/dev/null || echo dev)
+
+# ── 一键构建 ──
 build: build-server build-web
 
 build-server:
-	cd server && go build -o bin/backupx ./cmd/backupx
+	cd server && CGO_ENABLED=0 go build -trimpath -ldflags "-s -w -X main.version=$(VERSION)" -o bin/backupx ./cmd/backupx
 
 build-web:
 	cd web && npm run build
 
-# 开发模式（分别在两个终端运行）
+# ── 开发模式（分别在两个终端运行）──
 dev-server:
 	cd server && go run ./cmd/backupx
 
 dev-web:
 	cd web && npm run dev
 
-# 运行所有测试
+# ── 测试 ──
 test: test-server test-web
 
 test-server:
@@ -25,6 +28,14 @@ test-server:
 test-web:
 	cd web && npm run test
 
-# 清理构建产物
+# ── Docker 构建 ──
+docker:
+	docker build --build-arg VERSION=$(VERSION) -t backupx:$(VERSION) -t backupx:latest .
+
+# 国内加速构建（使用国内镜像源）
+docker-cn:
+	docker build --build-arg VERSION=$(VERSION) --build-arg USE_CHINA_MIRROR=true -t backupx:$(VERSION) -t backupx:latest .
+
+# ── 清理 ──
 clean:
 	rm -rf server/bin web/dist