Update Docker image & LCG view

Once multithreading landed in master, we should

• Rebuild the Docker images to have the latest Geant4 and ROOT versions
• Make sure the Geant4 in Docker is built by default with MT enabled (and remove the currently existing specific MT version)
• Update the CI to the latest & greatest LCG view, currently LCG_99 with Geant4 10.7.p01