feat(staging): optimize staging build with local Maven packaging

- Add server/Dockerfile.dev for fast local builds (JRE-only, ~70MB)
- Modify staging target to build JAR locally before Docker image
- Update .dockerignore to allow target/ directory for Dockerfile.dev
- Build time reduced from 17+ minutes to ~4 seconds
- Reuses local Maven cache (~/.m2/repository) instead of downloading in Docker

Makefile

@@ -178,8 +178,9 @@ validate-release-config: ## 校验发布环境变量文件（默认 .env.release
 	./scripts/validate-release-config.sh .env.release
 
 staging: ## 构建并启动 staging 环境，运行 smoke test（混合模式：后端镜像 + 前端静态文件）
-	@echo "=== [1/5] Building backend Docker image ==="
-	docker build -t $(STAGING_SERVER_IMAGE) -f server/Dockerfile server
+	@echo "=== [1/5] Building backend JAR and Docker image ==="
+	cd server && ./mvnw package -DskipTests -B -q
+	docker build -t $(STAGING_SERVER_IMAGE) -f server/Dockerfile.dev server
 	@echo "=== [2/5] Building frontend static files ==="
 	cd web && pnpm run build
 	@echo "=== [3/5] Starting dependency services ==="

server/.dockerignore

@@ -1,4 +1,7 @@
-**/target/
+# Exclude target for multi-stage build (Dockerfile)
+# But allow it for runtime build (Dockerfile.runtime) which needs pre-built JAR
+# **/target/
+
 **/.idea/
 **/*.iml
 .git/

server/Dockerfile.dev

@@ -0,0 +1,19 @@
+# Development Dockerfile
+# Expects pre-built JAR at skillhub-app/target/*.jar
+# Used by: make staging (local development workflow)
+FROM eclipse-temurin:21-jre-alpine
+
+RUN addgroup -S app && adduser -S app -G app
+WORKDIR /app
+
+# Copy pre-built JAR from local build (relative to server/ directory)
+COPY ./skillhub-app/target/*.jar app.jar
+
+RUN chown -R app:app /app
+USER app
+
+EXPOSE 8080
+HEALTHCHECK --interval=10s --timeout=3s \
+  CMD wget -qO- http://localhost:8080/actuator/health || exit 1
+
+ENTRYPOINT ["java", "-XX:MaxRAMPercentage=75.0", "-jar", "app.jar"]