Building scala project with mozilla/sbt image fails on 'Class not found exception'

All of a sudden our project cannot be built anymore (test step fails) because of following error:
java.lang.NoClassDefFoundError: scala/collection/GenTraversableOnce

Solved the problem not by changing image to hseeberger/scala-sbt:8u302_1.5.5_2.13.6, but by including hidden folders into the cache …