fix: build JAR locally, use minimal JRE image

Dockerfile

@@ -1,16 +1,8 @@
-# Multi-stage build with Chinese registry mirror
-FROM registry.cn-hangzhou.aliyuncs.com/chenxipan/maven:3.9-eclipse-temurin-17 AS builder
-
-WORKDIR /app
-COPY pom.xml .
-COPY src ./src
-RUN mvn clean package -DskipTests
-
-# Production stage
+# Production image - JAR is pre-built locally
 FROM eclipse-temurin:17-jre-alpine
 
 WORKDIR /app
-COPY --from=builder /app/target/*.jar app.jar
+COPY target/*.jar app.jar
 
 EXPOSE 8080