Main class for docker image should be configured from the application block

[e5l]

Having:

application {
    mainClass = "io.ktor.server.netty.EngineMain"

    val isDevelopment: Boolean = project.ext.has("development")
    applicationDefaultJvmArgs = listOf("-Dio.ktor.development=$isDevelopment")
}

Gives:

./gradlew buildImage
> Task :jibBuildTar FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':jibBuildTar'.
> com.google.cloud.tools.jib.plugins.common.MainClassInferenceException: Main class was not found, perhaps you should add a `mainClass` configuration to jib