並列、かつ指定したprojectだけdocker:publishする

build.sbt

import sbt.complete.DefaultParsers._
import sbt.complete.Parser

val dockerPublish = taskKey[Unit]("ダミーのタスク。本来sbt-native-packagerのやつ")
val dockerPublishParallel = inputKey[Unit]("今回追加するtask")

lazy val commonSettings = Def.settings(
  dockerPublish := {
    import java.time.LocalTime

    println(s"${LocalTime.now()} ${name.value} のpublish開始")
    // 並列で実行されるのがわかりやすいように、途中sleepする
    Thread.sleep(3000)
    println(s"${LocalTime.now()} ${name.value} のpublish終了")
  }
)

val allProjects = Seq[ProjectReference](a, b, c)

def distinctParser(exs: Set[String]): Parser[Seq[String]] = {
  val base = token(Space) ~> token(NotSpace examples exs)
  base.flatMap { ex =>
    val (_, notMatching) = exs.partition(GlobFilter(ex).accept)
    distinctParser(notMatching).map { result => ex +: result }
  } ?? Nil
}

dockerPublishParallel := InputTask.createDyn(
  Def.setting{ (_: State) =>
    val projectIds = allProjects.collect{ case p: LocalProject => p.project }.toSet
    distinctParser(projectIds)
  }
){
  Def.task(
    (projects: Seq[String]) => {
      // TODO projects.isEmpty なら逆に全部publish実行、みたいにしておいてもいいかも？

      projects.map { p =>
        dockerPublish in LocalProject(p)
      }.join
    }
  )
}.evaluated

lazy val a = project
  .settings(
    commonSettings
  )

lazy val b = project
  .settings(
    commonSettings
  )

lazy val c = project
  .settings(
    commonSettings
  )