Extended docs regarding native build with separate profile.

Author: ulischulte

spring-boot-admin-samples/spring-boot-admin-sample-servlet-graalvm/Readme.md

@@ -15,7 +15,7 @@ $ mvn clean package -Pnative
 ## Build an OCI image that can be run with Docker
 
 ```
-$ mvn spring-boot:build-image -Dspring-boot.build-image.imageName=spring-boot-admin-sample-servlet-graalvm:latest
+$ mvn spring-boot:build-image -Pnative -Dspring-boot.build-image.imageName=spring-boot-admin-sample-servlet-graalvm:latest
 ```
 
 ## Running the example