feat: Dockerfile 작성

Dockerfile

+FROM gradle:jdk21 AS build
+WORKDIR /home/gradle/project
+COPY --chown=gradle:gradle . .
+RUN gradle clean bootJar --no-daemon
+
+FROM openjdk:21-jdk-slim
+WORKDIR /app
+COPY --from=build /home/gradle/project/build/libs/*.jar app.jar
+EXPOSE 8080
+ENTRYPOINT ["java", "-jar", "app.jar"]