Merge pull request spring-projects#18932 from glours

* pr/18932:
  Polish "Improve Deploying to Containers section"
  Improve Deploying to Containers section

Closes spring-projectsgh-18932

Author: snicoll

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

@@ -35,19 +35,32 @@ 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
+	FROM openjdk:8-jdk-alpine AS builder
+	WORKDIR target/dependency
+	ARG appjar
+	COPY ${appjar} app.jar
+	RUN jar -xf ./app.jar
+
+	FROM openjdk:8-jre-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
+	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"]
 ----
 
+Assuming the above `Dockerfile` is the current directory, your docker image can be built specifying the path to your application jar, as show in the following example:
+
+[indent=0]
+----
+	docker build --build-arg appjar=path/to/myapp.jar .
+----
+
 
 
 [[cloud-deployment]]