Fix Online build, pin LCG 96b and Gaudi v33r1

New features:

• Force LCG version from configuration and pin to LCG 96b
• Update Gaudi version to v33r1
• Pin version of Detector to v0r4

Fixes:

• Improve parsing of CMakeLists (fixes Online & Allen build)

Improvements:

• Add HEP_OSlibs to prerequisites

Internal changes:

• Use self hosted docker image with python3
• Remove old readme