update dockerfile

examples/java/Dockerfile

@@ -1,5 +1,22 @@
-FROM amazoncorretto:21
-WORKDIR /app
+FROM alpine:latest as builder
+RUN sed -i s/v3.18/edge/ /etc/apk/repositories
+
+RUN apk upgrade --no-cache
+RUN apk --no-cache add openjdk20
+
+WORKDIR /workdir
 COPY . .
+
 RUN javac -cp .:lib/gson-2.10.1.jar ./planetwars/*.java Example.java
+
+FROM alpine:latest
+RUN sed -i s/v3.18/edge/ /etc/apk/repositories
+
+RUN apk --no-cache add openjdk20-jre
+RUN apk upgrade --no-cache
+
+WORKDIR /workdir
+
+COPY --from=builder /workdir /workdir
+
 CMD java -cp .:lib/gson-2.10.1.jar Example