Pro Tips from CI testing stage with docker

I am LOVING this stuff, having great progress quickly! Just fun… So i have created a docker runner for CI and need to be able to spin it up (which I got) and then copy a config to it and test the build… Where do I start with this? Most specifically the pushing of the cfg to the container and then run a command to verify sanity. I can build the container image I need, just need a nudge with gitlab CI interacting with the container, i.e. copying cfg, running command and then returning the result (exit code).