Specify docker volumes for Services

Hello!

We use Services for our build pipeline. In particular to run some integration tests. To use one feature I have to mount /var/run/docker.sock into container. However, I didn't find out how to do it.

Reference https://docs.gitlab.com/ee/ci/services/index.html#available-settings-for-services doesn't give a way to specify volume. Only name, alias, entrypoint, command and environment variables. How to put volume there? Or any other custom docker command-line arguments?