Integrate sbt Native Packager docker image creation

[elleFlorio]

The docker image defined in the /docker folder is good for development purposes. It would be good to integrate the sbt plugin for sbt Native Packager, in order to create a docker container running the Scalachain node binary. The image should have the correct name - elleflorio/scalachain - and the correct tags.

This issue can be implemented inside the docker-integration branch.