Fix #947 Correct scope for packageName in DockerPlugin (#948)

Author: muuki88

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

@@ -77,7 +77,7 @@ object DockerPlugin extends AutoPlugin {
       dockerExposedUdpPorts := Seq(),
       dockerExposedVolumes := Seq(),
       dockerRepository := None,
-      dockerAlias := DockerAlias(dockerRepository.value, None, packageName.value, Some((version in Docker).value)),
+      dockerAlias := DockerAlias(dockerRepository.value, None, (packageName in Docker).value, Some((version in Docker).value)),
       dockerUpdateLatest := false,
       dockerEntrypoint := Seq("bin/%s" format executableScriptName.value),
       dockerCmd := Seq(),