This is how you register gitlab runner to run in docker

launch_gitlab_runner_docker_executor.sh

#!/usr/bin/env bash

token=$1
url=$2
name=$3
docker_image_version="docker:dind"
tags=$4
docker_user="root"

docker run --rm -it -v /data/gitlab-runner/config:/etc/gitlab-runner gitlab/gitlab-runner register \
        -n \
        --url "${url}" \
        --registration-token "${token}" \
        --executor "docker" \
        --description "${name}" \
        --docker-image "${docker_image_version}" \
        --docker-volumes /cache \
        --docker-volumes /builds:/builds \
        --docker-volumes /var/run/docker.sock:/var/run/docker.sock \
        --docker-privileged \
        --docker-user "${docker_user}" \
        --tag-list "${tags}" \
        --paused \
        --locked="false" \
        --access-level="not_protected"

docker run -d --name gitlab-runner --restart always gitlab/gitlab-runner

docker run -d --name gitlab-runner --restart always -v /data/gitlab-runner/config:/etc/gitlab-runner gitlab/gitlab-runner

docker run -d --name gitlab-runner --restart always -v /data/gitlab-runner/config:/etc/gitlab-runner -v /var/run/docker.sock:/var/run/docker.sock gitlab/gitlab-runner

CI_PROJECT_DIR answers the following formula:

/builds/${CI_RUNNER_SHORT_TOKEN}/${CI_CONCURRENT_PROJECT_ID}/${CI_PROJECT_PATH}

https://docs.gitlab.com/runner/configuration/advanced-configuration.html#default-build-directory