How to debug Gitlab code?

I’m a security researcher and i would like to be able to debug Gitlab’s code, this is the first time i’m doing this for sudh a large project so i’m wondering if its even possible.
I usually spin up the version i want of Gitlab in a docker container, is there a way i could attach a debugger to this container and debug the code line by line ? obviously not the whole codebase but for example the create issue functionality, i would like to debug the source code step by step from the handling of user input to the redirection to the issue link.
Is this kind of debugging possible in Vscode or rubymine? if so please tell me what extensions i need to install or refer me to any guildes

Suggest starting your research in the development documentation.