Improve Dockerfile example in deployment documentation

Add stage build to unpack jar
Change runtime base image by a jre-alpine

Signed-off-by: Guillaume Lours <guillaume.lours@docker.com>

spring-boot-project/spring-boot-docs/src/main/asciidoc/deployment.adoc

@@ -35,17 +35,23 @@ Once you have unpacked the jar file, you can also get an extra boost to startup
 
 More efficient container images can also be created by copying the dependencies to the image as a separate layer from the application classes and resources (which normally change more frequently).
 There is more than one way to achieve this layer separation.
-For example, using a `Dockerfile` you could express it in this form (assuming the jar is already unpacked at `target/dependency`):
+For example, using a `Dockerfile` you could express it in this form:
 
 [indent=0]
 ----
-	FROM openjdk:8-jdk-alpine
-	VOLUME /tmp
-	ARG DEPENDENCY=target/dependency
-	COPY ${DEPENDENCY}/BOOT-INF/lib /app/lib
-	COPY ${DEPENDENCY}/META-INF /app/META-INF
-	COPY ${DEPENDENCY}/BOOT-INF/classes /app
-	ENTRYPOINT ["java","-cp","app:app/lib/*","com.example.MyApplication"]
+    FROM openjdk:8-jdk-alpine AS builder
+    WORKDIR target/dependency
+    COPY ./target/*.jar .
+    RUN jar -xf ./*.jar
+
+    FROM openjdk:8-jre-alpine
+    EXPOSE 8080
+    VOLUME /tmp
+    ARG DEPENDENCY=target/dependency
+    COPY --from=builder ${DEPENDENCY}/BOOT-INF/lib /app/lib
+    COPY --from=builder ${DEPENDENCY}/META-INF /app/META-INF
+    COPY --from=builder ${DEPENDENCY}/BOOT-INF/classes /app
+    ENTRYPOINT ["java","-cp","app:app/lib/*","com.example.MyApplication"]
 ----