Additional docker commands

Yea, I almost wish sbt-docker didn't want the arguments as a [`String: _*`](https://github.com/marcuslonnberg/sbt-docker/blob/master/src/main/scala/sbtdocker/DockerfileCommands.scala#L102) only to then [`.mkString(" ")`](https://github.com/marcuslonnberg/sbt-docker/blob/master/src/main/scala/sbtdocker/Instructions.scala#L6) later on.