Improve font configuration
Improve font configuration

Due to some standard fonts missing in the SBT runner image,
the JVM was failing to load the fonts when running any font-related code.
DejaVu fonts are added to mitigate this, as per:

  https://github.com/docker-library/openjdk/issues/73#issuecomment-207816707

Additionally, the Source Code Pro fonts are added. They look somewhat better
and are used by [reftree](https://github.com/stanch/reftree).

scalacenter/scastie,scalacenter/scastie,OlegYch/scastie,OlegYch/scastie,OlegYch/scastie,scalacenter/scastie