Add Docker labels easily

[NeQuissimus]

This change allows for an easy way to add labels to Docker images.
It was previously possible to add this as custom commands but given the importance of labels, it seems reasonable to make adding them more easily.

I believe there is not much documentation necessary. I added a reference to the settings key.

As an example, we will be using this to forward Jenkins job variables into our Docker images.

dockerLabels := Map(
  "BUILD" -> sys.env.getOrElse("BUILD_NUMBER", "local"),
  "GIT_COMMIT" -> sys.env.getOrElse("GIT_COMMIT", "local")
)

[muuki88]

Thanks a lot for your pull request. Feature and implementation look solid to me.

Could you add a small test, setting some labels that ensure that the docker command still runs?

You can find the tests here:
https://github.com/sbt/sbt-native-packager/tree/master/src/sbt-test/docker

You can find some docs on how to run the tests in the Developer Guide

Also +1 for the documentation :)

[muuki88]

These look like formatting changes. When you run sbt scalafmt it should format correctly.
Or if you use intellij along with the scalafmt plugin you may use a newer or older version.

[NeQuissimus]

I had forgotten about a test :)

I ran scalafmt against everything, seems there were a few files it was unhappy with...

[muuki88]

Thanks a lot :)