root path moved - suggest to set ROOTSYS in docker container
Due to the clean-up to solve #11 (closed), ROOT was moved from /opt/root
to /usr/local/bin
This causes the pipeline in Jpp to fail, as observed by @mdejong.
I propose to set the root environment correctly from within the container - including the environment variable, then we don't need to source the thisroot-script when depending on this container.