[full, cpp] add GDB, document how one can debug

[marcdumais-work]

Follow-up to #258

I expect we'll need to do a bit more work before C/C++ debugging works in theia-full and theia-cpp-docker:

• install GDB in the images
• document that one needs to start the docker image with a "special" CLI argument: --security-opt seccomp=unconfined, I think. Maybe more will be needed as well.

For theia-cpp-electron, we could add to the README that one needs GDB installed on the system and in the path.