Scala-sbt builder needs care

If someone's interested in a Scala builder for Cloud Build, let's get in touch.

There are multiple problems with the current one, the biggest maybe that it fails to load in the libraries that make `sbt` and Scala (it's kind of the beef, right?).

I had a go at my own approach, but it needs some testing. You can find it here: https://github.com/akauppi/sbt-scala-builder

Thanks for the help.