Dockerfile: fix base image no longer working

[pataquets]

Dockerfile was no longer working, since base image no longer has a :latest tag (the default). It also is deprecated in favor of https://hub.docker.com/r/sbtscala/scala-sbt/ However, it doesn't support the :latesttag, so you have to use specific version tags. See docs for more info. I've picked the one using Jammy (latest Ubuntu LTS) and JDK8.