Initialise sbt in docker image

[markus1189]

On the first execution, sbt downloads some scala jars and on first
compile it also compiles the compiler-bridge:

[info] downloading https://repo1.maven.org/maven2/org/scala-lang/scala-library/2.12.9/scala-library-2.12.9.jar ...
[info] downloading https://repo1.maven.org/maven2/org/scala-lang/scala-compiler/2.12.9/scala-compiler-2.12.9.jar ...                                                                                                
[info] downloading https://repo1.maven.org/maven2/org/scala-lang/scala-reflect/2.12.9/scala-reflect-2.12.9.jar ...                                                                                                  
[info] downloading https://repo1.maven.org/maven2/org/fusesource/jansi/jansi/1.12/jansi-1.12.jar ...
[info]  [SUCCESSFUL ] org.scala-lang#scala-reflect;2.12.9!scala-reflect.jar (454ms)                                                                                                                                 
[info]  [SUCCESSFUL ] org.fusesource.jansi#jansi;1.12!jansi.jar (446ms)                                   
[info]  [SUCCESSFUL ] org.scala-lang#scala-library;2.12.9!scala-library.jar (1096ms)                      
[info]  [SUCCESSFUL ] org.scala-lang#scala-compiler;2.12.9!scala-compiler.jar (1518ms)
[info] Done updating.                                                                                     
[info] Compiling 1 Scala source to /tmp/tmp.hrfM92SWVI/target/scala-2.12/classes ...
[info] Non-compiled module 'compiler-bridge_2.12' for Scala 2.12.9. Compiling...                                                                                                                                    
[info]   Compilation completed in 8.037s.

This adds the necessary steps to already do this in the docker image, so CI doesn't have to do
it every time.