Fixes #18 - Add Dockerfile for multi stage builds

[haxorof]

Simply explained this is a feature introduced in Docker v17.06 which basically makes it easier to create images with smaller size etc.

In the Dockerfile.multi I created it first build the application in one container and just takes the compiled binaries and put it into another. This thing I did before with releases on Github and fetches those will basically be redundant as soon as this feature is available at Docker Hub. 😄

[bruth]

@haxorof Thanks. That is a handy feature