build: multi stage docker build

Dockerfile

@@ -1,19 +1,36 @@
-# After spending lots of time in Docker hell I don't have any more apetite for
-# this. I'm lost in a sea of different JRE/JDK distributions with strange names,
-# and they all give me different linker errors (at runtime!) it's completely
-# impossible to debug. God, I hate the JVM.
-# If someone who actually knows what they're doing comes around, it'd be great
-# to separate this out to a builder/runner staged build. This leads to a
-# horrendously large image.
+# Example of custom Java runtime using jlink in a multi-stage container build
+FROM eclipse-temurin:11 as jre-build
+
+# Create a custom Java runtime
+RUN $JAVA_HOME/bin/jlink \
+         --add-modules java.base \
+         --add-modules java.logging \
+         --strip-debug \
+         --no-man-pages \
+         --no-header-files \
+         --compress=2 \
+         --output /javaruntime
+
+# Stage 1: Build stage
 FROM gradle:7-jdk11 AS build
 
 WORKDIR /workdir
 
 COPY . .
-RUN gradle build
+RUN gradle fatJar
+
+# Stage 2: Runtime stage
+# Needs to be a recent ish debian image to have a new enough
+# libc. Still haven't understood the finer points here. 
+FROM debian:bookworm-slim
+
+ENV JAVA_HOME=/opt/java/openjdk
+ENV PATH "${JAVA_HOME}/bin:${PATH}"
+COPY --from=jre-build /javaruntime $JAVA_HOME
+
+WORKDIR /app
 
-# TODO: parameterize the version?
-RUN unzip build/distributions/deskriptor-1.0-SNAPSHOT.zip && \
-    cp -r /workdir/deskriptor-1.0-SNAPSHOT /deskriptor
+COPY --from=build /workdir/build/libs/deskriptor-1.0-SNAPSHOT-standalone.jar /app/
+COPY logging.properties /logging.properties
 
-ENTRYPOINT [ "/deskriptor/bin/deskriptor"]
+ENTRYPOINT [ "java", "-jar", "/app/deskriptor-1.0-SNAPSHOT-standalone.jar" ]

build.gradle.kts

@@ -68,4 +68,21 @@ sourceSets["main"].java {
 
 application {
     mainClass.set("no.bb.deskriptor.DeskriptorKt")
-}
+}
+
+tasks {
+    val fatJar = register<Jar>("fatJar") {
+        dependsOn.addAll(listOf("compileJava", "compileKotlin", "processResources")) // We need this for Gradle optimization to work
+        archiveClassifier.set("standalone") // Naming the jar
+        duplicatesStrategy = DuplicatesStrategy.EXCLUDE
+        manifest { attributes(mapOf("Main-Class" to application.mainClass)) } // Provided we set it up in the application plugin configuration
+        val sourcesMain = sourceSets.main.get()
+        val contents = configurations.runtimeClasspath.get()
+            .map { if (it.isDirectory) it else zipTree(it) } +
+                sourcesMain.output
+        from(contents)
+    }
+    build {
+        dependsOn(fatJar) // Trigger fat jar creation during build
+    }
+}