Add instructions for setting up [runners.docker] section of gitlab-runner config.toml

Need to be sure to add the following section to the guide on setting up a runner:

[runners.docker]
    tls_verify = false
    image = "docker:26.0.1"
    privileged = true
    disable_entrypoint_overwrite = false
    oom_kill_disable = false
    disable_cache = false
    volumes = ["/cache", "/certs/client", "/var/run/docker.sock:/var/run/docker.sock"]
    shm_size = 0
    network_mtu = 0
    environment = ["DOCKER_CLEANUP_INTERVAL=24h"]

privileged = True and volumes = [..., "/var/run/docker.sock:/var/run/docker.sock"] are especially important since these are not the default settings. In particular, setting the docker.sock volume is necessary for the gitlab-runner service to access docker correctly, and privileged allows you to use the dind service