fix(docker): 移除 BuildKit cache mount，兼容无 buildx 的标准 Docker

--mount=type=cache 语法需要 BuildKit/buildx 支持，
生产服务器无 buildx 插件导致每次构建实际使用旧缓存层。
移除该指令后可用标准 docker build 正常编译。

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile

@@ -1,4 +1,3 @@
-# syntax=docker/dockerfile:1.7
 ARG SERVICE_MODULE=tenant-service
 
 FROM --platform=linux/amd64 maven:3.9.9-eclipse-temurin-21 AS build
@@ -16,8 +15,7 @@ COPY update-service ./update-service
 COPY demo-service ./demo-service
 COPY file-service ./file-service
 
-RUN --mount=type=cache,target=/root/.m2,sharing=locked \
-    mvn -U -s /workspace/maven-settings.xml -pl ${SERVICE_MODULE} -am -DskipTests package
+RUN mvn -U -s /workspace/maven-settings.xml -pl ${SERVICE_MODULE} -am -DskipTests package
 
 FROM --platform=linux/amd64 eclipse-temurin:21-jre-jammy
 WORKDIR /app