Tweak the Dockerfile to allow for caching of sbt downloaded dependencies between builds

I've been making some other changes to Marathon and using Docker to create the builds is pretty slow without this change.  Previously every `docker build -t marathon-tip .` would take 10-15 minutes just to download sbt dependencies.  This now downloads and caches those dependencies unless a change is made to the version.sbt or project/ of the project.  This gets builds down to a couple minutes.