Add Dockerfiles for service and base images

backend/Dockerfile.base

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+# Dockerfile.base
+FROM eclipse-temurin:21-jre-alpine
+
+WORKDIR /shared
+
+COPY backend/shared/target/shared-*.jar shared.jar

backend/Dockerfile.service

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+# Dockerfile.service
+FROM your-registry/shared-base:latest
+
+WORKDIR /app
+ARG JAR_FILE=app.jar
+COPY backend/gateway/target/gateway-*.jar app.jar
+
+# Use JarLauncher and include shared.jar in classpath
+ENTRYPOINT ["java", "-cp", "/shared/shared.jar:/app/app.jar", "org.springframework.boot.loader.JarLauncher"]