Install gitlab runner, docker version: https://docs.gitlab.com/runner/install/docker.html

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

Register the runner:

docker run --rm -t -i -v /srv/gitlab-runner/config:/etc/gitlab-runner gitlab/gitlab-runner register

# coorinator URL:
https://gitlab.research.example.com

# gitlab-ci token
# use the token from the project, ex: https://gitlab.research.example.com/auto/helloworld/-/settings/ci_cd
# or from the GROUP to run them all: https://gitlab.research.example.com/groups/auto/-/settings/ci_cd

# description
javabuilder

# tags
webapps

# executor
docker

# image
docker:19.03.1

Then edit the config.toml and set priviledged=true

sudo nano /srv/gitlab-runner/config/config.toml

and restart the gitlab-runner container

docker ps
docker stop fcb.....b3ef
docker start fcb.....b3ef
docker logs -f fcb.....b3ef

Watch the log

docker logs gitlab-runner --follow

The resulting runner configuration, and an example config.toml where docker connects via socket:

sudo cat /srv/gitlab-runner/config/config.toml

concurrent = 1
check_interval = 0

[session_server]
  session_timeout = 1800

[[runners]]
  name = ""
  url = ""
  token = ""
  executor = ""

[[runners]]
  name = "java-runner-webapps"
  url = "https://gitlab.research.example.com"
  token = "XXXXXXXXXXXXXXXXXXX"
  executor = "docker"
  output_limit = 32096
  [runners.custom_build_dir]
  [runners.docker]
    tls_verify = false
    image = "docker:19.03.1"
    privileged = true
    disable_entrypoint_overwrite = false
    oom_kill_disable = false
    disable_cache = false
    volumes = ["/var/run/docker.sock:/var/run/docker.sock", "/cache", "/opt/mymavenrepo:/root/m2"]
    shm_size = 0
  [runners.cache]
    [runners.cache.s3]
    [runners.cache.gcs]