How to use Gitlab CI/CD docker image

Hi there,
I am new to Gitlab, i have installed gitlab on my localserver and install & configure runner on another server, i have also cloned a demo project on this server and when i commit after making any changes in it creates a docker image on the server where i have install gitlab runner.

Now my question is when i ran container from that docker image , and when i get inside the container, i didn’t find any file gitlab project file there.

so may i k now what is the use of that docker image which was created after every commit.?
also please let me know if there any document for beginner