convert dockerVersion from a setting to a task (#1058)

Author: dwickern

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

@@ -84,7 +84,7 @@ object DockerSpotifyClientPlugin extends AutoPlugin {
     }
   }
 
-  def dockerServerVersion: Def.Initialize[Option[DockerVersion]] = Def.setting {
+  def dockerServerVersion: Def.Initialize[Task[Option[DockerVersion]]] = Def.task {
     val docker: DockerClient = DefaultDockerClient.fromEnv().build()
     DockerVersion.parse(docker.version().version())
   }

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

@@ -29,7 +29,7 @@ trait DockerKeys {
     "Docker CMD. Used together with dockerEntrypoint. Arguments passed in exec form"
   )
   val dockerExecCommand = SettingKey[Seq[String]]("dockerExecCommand", "The shell command used to exec Docker")
-  val dockerVersion = SettingKey[Option[DockerVersion]]("dockerVersion", "The docker server version")
+  val dockerVersion = TaskKey[Option[DockerVersion]]("dockerVersion", "The docker server version")
   val dockerBuildOptions = SettingKey[Seq[String]]("dockerBuildOptions", "Options used for the Docker build")
   val dockerBuildCommand = SettingKey[Seq[String]]("dockerBuildCommand", "Command for building the Docker image")
   val dockerLabels = SettingKey[Map[String, String]]("dockerLabels", "Labels applied to the Docker image")