DockerBuildPlugin should obey envVars

[ckarenz]

I have code like the following in my web service's build.sbt, so I can define the port in one place:

containerPort := 8087 // For jetty:start
dockerPorts += containerPort.value // For dockerRun
envVars += ("PORT" -> containerPort.value.toString) // For run

Later, in code, I have:

sys.env.get("PORT").map(_.toInt).getOrElse(8080)

And finally, I need a duplicate directive in the Dockerfile to ensure the service binds itself to the correct port when running in a container:

ENV PORT 8087

I'd very much like envVars to get picked up by the docker plugin, similarly to javaOptions. This would allow me to avoid the duplication in our Dockerfile.