Update Docker stack
This MR updates our Docker image with
- devtoolset 8, i.e. GCC 8.3.1
- Geant4 10.6.p02 (latest stable)
- ROOT 6.22.02 (latest stable)
- LCIO 2.15.3 (latest stable)
Reminder to self: update the base and deps Docker images manually when approving/merging this.