There is no substitute for poking around, so if you need to get into a Docker container, run it with "interactive" and "tty" options:
docker run -it <image> /bin/bash
Pro tip: to keep a container running so you can login and poke around, add a sleep loop command to your script:
test_it:
script:
- /bin/echo I am a pretend test suite. I passed!
- while true; do sleep 1; date; done
Now you can run docker ps to get the container id, and start an interactive session with docker exec -it <id> /bin/bash.