optimize docker layering

src/main/scala/com/typesafe/sbt/packager/docker/DockerPlugin.scala

@@ -110,14 +110,7 @@ object DockerPlugin extends AutoPlugin {
       implicit val conv: FileConverter = conv0
       val dockerBaseDirectory = (Docker / defaultLinuxInstallLocation).value
       // Ensure this doesn't break even if the JvmPlugin isn't enabled.
-      var artifacts = projectDependencyArtifacts.?.value.getOrElse(Nil).map(_.data).toSet
-
-      // add the classpath jar to the project artifacts to improve layer caching as it is created by
-      // the ClasspathJarPlugin and is not part of the projectDependencyArtifacts
-      ClasspathJarPlugin.autoImport.packageJavaClasspathJar.?.value match {
-        case Some(p) => artifacts += p
-        case _       =>
-      }
+      val dependencies = (Runtime / dependencyClasspath).value.map(_.data).toSet
 
       val oldFunction = dockerLayerGrouping.value
 
@@ -130,12 +123,13 @@ object DockerPlugin extends AutoPlugin {
       val confDir = dockerBaseDirectory + "/conf/"
 
       oldPartialFunction.orElse {
-        // bin directory contains start scripts which are containing artifacts / classpath jar,
+        // bin directory contains start scripts which reference artifacts / classpath jar,
         // so should be together with actual artifacts
-        case (file, path) if artifacts(file) || path.startsWith(binDir) => 4
-        case (_, path) if path.startsWith(jreDir)                       => 3
-        case (_, path) if path.startsWith(libDir)                       => 2
-        case (_, path) if path.startsWith(confDir)                      => 1
+        case (_, path) if path.startsWith(jreDir)  => 10
+        case (file, _) if dependencies(file)       => 20
+        case (_, path) if path.startsWith(confDir) => 30
+        case (_, path) if path.startsWith(libDir)  => 40
+        case (_, path) if path.startsWith(binDir)  => 40
       }
     },
     dockerAliases := {

src/sbt-test/docker/test-layer-groups-playframework/build.sbt

@@ -1,4 +1,4 @@
-name := """test-layer-groups-playframework"""
+name := "test-layer-groups-playframework"
 organization := "com.example"
 
 version := "1.0-SNAPSHOT"
@@ -15,3 +15,40 @@ libraryDependencies += "org.scalatestplus.play" %% "scalatestplus-play" % "7.0.1
 
 // Adds additional packages into conf/routes
 // play.sbt.routes.RoutesKeys.routesImport += "com.example.binders._"
+
+enablePlugins(DockerPlugin, LauncherJarPlugin)
+
+// generate a conf/application.ini
+Universal / javaOptions ++= Seq("-J-Xms1024m", "-J-Xmx1024m")
+
+TaskKey[Unit]("checkDockerLayers") := {
+  val layers = (Docker / dockerLayerMappings).value.groupBy(_.layerId).withDefaultValue(Nil)
+
+  val layer10 = layers(Some(10))
+  assert(layer10.isEmpty, "layer 10 is empty because jlink is not used")
+
+  val layer20 = layers(Some(20))
+  assert(
+    layer20.forall(_.file.getPath.startsWith(csrCacheDirectory.value.getPath)),
+    "layer 20 only contains external libraries"
+  )
+
+  val layer30 = layers(Some(30))
+  assert(layer30.exists(_.path == "/opt/docker/conf/conf/application.ini"), "layer 30 contains application.ini")
+  assert(layer30.exists(_.path == "/opt/docker/conf/conf/logback.xml"), "layer 30 contains logback.xml")
+
+  val layer40 = layers(Some(40))
+  assert(
+    layer40.exists(_.file == (Runtime / PlayKeys.playJarSansExternalized).value),
+    "layer 40 contains Play's -sans-externalized.jar"
+  )
+  assert(layer40.exists(_.file == PlayKeys.playPackageAssets.value), "layer 40 contains Play's -assets.jar")
+  assert(layer40.exists(_.file == packageJavaLauncherJar.value), "layer 40 contains launcher jar")
+  assert(
+    layer40.exists(layer => makeBashScripts.value.map(_._1).contains(layer.file)),
+    "layer 40 contains start scripts"
+  )
+
+  val layerFinal = layers(None)
+  assert(layerFinal.exists(_.path.startsWith("/opt/docker/share/doc/api/")), "final layer contains API docs")
+}

src/sbt-test/docker/test-layer-groups-playframework/project/plugins.sbt

@@ -1,2 +1,3 @@
 addSbtPlugin("org.playframework" % "sbt-plugin" % "3.0.6")
 addSbtPlugin("org.foundweekends.giter8" % "sbt-giter8-scaffold" % "0.17.0")
+addSbtPlugin("com.github.sbt" % "sbt-native-packager" % sys.props("project.version"))

src/sbt-test/docker/test-layer-groups-playframework/test

@@ -0,0 +1 @@
+> checkDockerLayers