It will be nice to have docker-compose ready on our runner so people could use it effortless.
There are some approach that installs docker-compose in the .gitlab-ci.yml before_script step to so the next steps could use it, but I personally think it is a bit overwhelming and maybe we could provide the runner this capacity natively.
To do so, we just have to change the --docker-image "docker:stable" to use another image that has docker-compose installed. I found this guy that has published this image that could be used: https://hub.docker.com/r/tmaier/docker-compose/
I tested it and worked well...

What do you think?