fix: resolve stale class conflict in Docker build by adding clean and .dockerignore

dap-tool-email/Dockerfile

@@ -1,4 +1,4 @@
-FROM eclipse-temurin:21-jdk-alpine AS builder
+FROM eclipse-temurin:21-jdk-alpine AS builder
 WORKDIR /app
 COPY gradlew .
 COPY gradle gradle
@@ -7,7 +7,7 @@ COPY build.gradle settings.gradle ./
 COPY dap-tool-core dap-tool-core
 COPY dap-tool-email dap-tool-email
 RUN chmod +x gradlew
-RUN ./gradlew :dap-tool-email:build -x test
+RUN ./gradlew clean :dap-tool-email:build -x test
 
 FROM eclipse-temurin:21-jre-alpine
 WORKDIR /app
@@ -16,3 +16,4 @@ ENV TZ=Asia/Seoul
 COPY --from=builder /app/dap-tool-email/build/libs/*-SNAPSHOT.jar app.jar
 EXPOSE 8083
 ENTRYPOINT ["java", "-jar", "app.jar"]
+