SBT: How to Dockerize a fat jar? - sbt

Question

I'm building a Docker image with a fat jar. I use the sbt-assembly plugin to build the jar, and the sbt-native-packager to build the Docker image. I'm not very familiar with SBT and am running into the following issues.
I'd like to declare a dependency on the assembly task from the docker:publish task, such that the fat jar is created before it's added to the image. I did as instructed in the doc, but it's not working. assembly doesn't run until I invoke it.
publish := (publish dependsOn assembly).value
One of the steps in building the image is copying the fat jar. Since assembly plugin creates the jar in target/scala_whatever/projectname-assembly-X.X.X.jar, I need to know the exact scala_whatever and the jar name. assembly seems to have a key assemblyJarName but I'm not sure how to access it. I tried the following which fails.
Cmd("COPY", "target/scala*/*.jar /app.jar")
Help!

Answer 1

Answering my own questions, the following works:
enablePlugins(JavaAppPackaging, DockerPlugin)
assemblyMergeStrategy in assembly := {
case x => {
val oldStrategy = (assemblyMergeStrategy in assembly).value
val strategy = oldStrategy(x)
if (strategy == MergeStrategy.deduplicate)
MergeStrategy.first
else strategy
}
}
// Remove all jar mappings in universal and append the fat jar
mappings in Universal := {
val universalMappings = (mappings in Universal).value
val fatJar = (assembly in Compile).value
val filtered = universalMappings.filter {
case (file, name) => !name.endsWith(".jar")
}
filtered :+ (fatJar -> ("lib/" + fatJar.getName))
}
dockerRepository := Some("username")
import com.typesafe.sbt.packager.docker.{Cmd, ExecCmd}
dockerCommands := Seq(
Cmd("FROM", "username/spark:2.1.0"),
Cmd("WORKDIR", "/"),
Cmd("COPY", "opt/docker/lib/*.jar", "/app.jar"),
ExecCmd("ENTRYPOINT", "/opt/spark/bin/spark-submit", "/app.jar")
)
I completely overwrite the docker commands because the defaults add couple of scripts that I don't need because I overwrite the entrypoint as well. Also, the default workdir is /opt/docker which is not where I want to put the fat jar.
Note that the default commands are shown by show dockerCommands in sbt console.