Use layered Spring Boot container building

Resolves #4.

Author: iay

.dockerignore

@@ -1,2 +1,3 @@
 .git/
 target/
+Dockerfile

Dockerfile

@@ -8,10 +8,10 @@
 #
 # Build the .jar file in a build container.
 #
-FROM maven:3.5.3-jdk-11 AS build-jar
+FROM maven:3.5.3-jdk-11 AS builder
 MAINTAINER Ian Young <[email protected]>
 
-WORKDIR /user
+WORKDIR /application
 COPY pom.xml ./
 COPY swagger swagger
 COPY src src
@@ -20,14 +20,39 @@ RUN mvn --batch-mode \
     -Dorg.slf4j.simpleLogger.log.org.apache.maven.cli.transfer.Slf4jMavenTransferListener=warn \
     package
 
+#
+# Extract the layers.
+#
+RUN java -Djarmode=layertools \
+    -jar target/md-validator-0.1.0-SNAPSHOT.jar \
+    extract
+
 #
 # Build the deployable image.
 #
 FROM amazoncorretto:11
 MAINTAINER Ian Young <[email protected]>
 
-WORKDIR /user
-COPY --from=build-jar /user/target/md-validator-0.1.0-SNAPSHOT.jar .
+#
+# Copy the layers extracted from the JAR.
+#
+WORKDIR /application
+COPY --from=builder application/dependencies/ ./
+COPY --from=builder application/spring-boot-loader/ ./
+COPY --from=builder application/snapshot-dependencies/ ./
+COPY --from=builder application/application/ ./
+
+#
+# At this point, we have:
+#
+# /application/org... containing the Spring Boot loader
+# /application/META-INF containing the application metadata
+# /application/BOOT-INF containing the application and its dependencies
+# /application/BOOT-INF/classes contain the application classes and resources
+#
+# To customise, add layers modifying /application/BOOT-INF/classes...
+#
 
 EXPOSE 8080
-CMD ["java", "-jar", "md-validator-0.1.0-SNAPSHOT.jar"]
+
+ENTRYPOINT ["java", "org.springframework.boot.loader.JarLauncher"]

pom.xml

@@ -266,13 +266,17 @@
             </plugin>
 
             <!--
-                Ask Spring Boot to enhance the resulting .jar so that it is
-                directly executable.
+                Build a directly executable, layered JAR.
             -->
             <plugin>
                 <groupId>org.springframework.boot</groupId>
                 <artifactId>spring-boot-maven-plugin</artifactId>
                 <version>${spring.boot.version}</version>
+                <configuration>
+                    <layers>
+                        <enabled>true</enabled>
+                    </layers>
+                </configuration>
                 <executions>
                     <execution>
                         <goals>