Where to find beginners info

My company is trying to move from Jenkins to gitlab . I need a source for good beginner learning material. Can someone please recommend?

I am trying to get our project back on track and hoping someone can point me at something that many people must have solved.

I have very strict regulated file system isolation access restrictions. We believe that using docker based runners will demonstrate to InfoSec that docker runners meet the file system isolation requirements. We are comfortable with shell runners. but due to isolation requirements only one project can use gitlab. I need to teach my people how to translate the shell runner into docker runner so that we can have more than 1 project building something