Improve Dockerfiles example in packaging documentation

spring-boot-project/spring-boot-docs/src/docs/antora/modules/reference/pages/packaging/container-images/dockerfiles.adoc

@@ -33,18 +33,30 @@ Here is an example of a Dockerfile using `jarmode`.
 
 [source,dockerfile]
 ----
-FROM bellsoft/liberica-runtime-container:jre-17-cds-slim-glibc as builder
+# Perform the extraction in a separate builder container
+FROM bellsoft/liberica-openjre-debian:17-cds AS builder
 WORKDIR /builder
+# This points to the built jar file in the target folder
+# Adjust this to 'build/libs/*.jar' if you're using Gradle
 ARG JAR_FILE=target/*.jar
+# Copy the jar file to the working directory and rename it to application.jar
 COPY ${JAR_FILE} application.jar
+# Extract the jar file using an efficient layout
 RUN java -Djarmode=tools -jar application.jar extract --layers --destination extracted
 
-FROM bellsoft/liberica-runtime-container:jre-17-cds-slim-glibc
+# This is the runtime container
+FROM bellsoft/liberica-openjre-debian:17-cds
 WORKDIR /application
+# Copy the extracted jar contents from the builder container into the working directory in the runtime container
+# Every copy step creates a new docker layer
+# This allows docker to only pull the changes it really needs
 COPY --from=builder /builder/extracted/dependencies/ ./
 COPY --from=builder /builder/extracted/spring-boot-loader/ ./
 COPY --from=builder /builder/extracted/snapshot-dependencies/ ./
 COPY --from=builder /builder/extracted/application/ ./
+# Start the application jar - this is not the uber jar used by the builder
+# This jar only contains application code and references to the extracted jar files
+# This layout is efficient to start up and CDS friendly
 ENTRYPOINT ["java", "-jar", "application.jar"]
 ----