ACTSFW-36 Add Gitlab continous integration

This adds continous integration for the test framework. The execution happens on a minimal Docker image based on CERN CentOS7 with the additional software taken from /cvmfs/sft.cern.ch/... using LCG release 88.

ACTS is compiled twice: once in its default configuration and once with all plugins enabled. To reduce output size and compilation time it is always build as Release and w/o the unit tests. The framework is then build against both versions.

Due to inconsistencies of the Geant4 installation paths in the LCG release, USE_GEANT4 is still disabled at the moment. See the corresponding LCG bug report

Fixes ACTSFW-36.