Use caps in docker file example for consistency

See gh-19029

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

@@ -41,8 +41,8 @@ For example, using a `Dockerfile` you could express it in this form:
 ----
 	FROM openjdk:8-jdk-alpine AS builder
 	WORKDIR target/dependency
-	ARG appjar
-	COPY ${appjar} app.jar
+	ARG APPJAR=target/*.jar
+	COPY ${APPJAR} app.jar
 	RUN jar -xf ./app.jar
 
 	FROM openjdk:8-jre-alpine
@@ -54,7 +54,7 @@ For example, using a `Dockerfile` you could express it in this form:
 	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:
+Assuming the above `Dockerfile` is the current directory, your docker image can be built with `docker build .`, or optionally specifying the path to your application jar, as show in the following example:
 
 [indent=0]
 ----
@@ -62,7 +62,6 @@ Assuming the above `Dockerfile` is the current directory, your docker image can
 ----
 
 
-
 [[cloud-deployment]]
 == Deploying to the Cloud
 Spring Boot's executable jars are ready-made for most popular cloud PaaS (Platform-as-a-Service) providers.