DockerPlugin doesn't work with multiple main classes by default

[kubukoz]

Expected behaviour

sbt Docker/publishLocal produces a runnable image or fails at build time in the presence of multiple main classes

Actual behaviour

The image fails startup

Information

• What sbt-native-packager are you using: 1.9.7
• What sbt version: 1.6.1
• What is your build system: sbt
• What package are you building docker
• What version has your build tool: ???
• What is your target system: macOS, also confirmed on Linux

Reproduction

1. Make an sbt project using NP:

val root = project
  .in(file("."))
  .enablePlugins(JavaAppPackaging)
  .enablePlugins(DockerPlugin)

2. Add multiple main classes

object Foo extends App
object Bar extends App

3. sbt Docker/publishLocal

4. Try to run the generated image:

$ docker run --rm -it sha256:46c53661a82306d95dbd9e9db28fa7e98d7f5ae4553d5c0091f0f71dfdbb8c97
docker: Error response from daemon: OCI runtime create failed: container_linux.go:380: starting container process caused: exec: "/opt/docker/bin/root": stat /opt/docker/bin/root: no such file or directory: unknown.

Docker/dockerEntrypoint points to /opt/docker/bin/root, and setting it to a specific binary helps. Clearly the names are set by a heuristic documented in the docs.

However, I don't think the Docker plugin's behavior is intuitive, because if you add a new main class to a working build you'll suddenly be unable to run the images.

Maybe Docker/stage should fail if there are many main classes and none of them is explicitly chosen (by Docker/mainClass or dockerEntrypoint, I'm not sure)?