Registrar o runner e criar o container para execução

docker run -it --name <container-name> \
  -v /var/run/docker.sock:/var/run/docker.sock \
  -v /srv/gitlab-runner/config:/etc/gitlab-runner \
  gitlab/gitlab-runner register

Após responder as questões informando endereço e token

Iniciar o container

docker start <container-name> 

Executar comandos do runner no container

docker exec -it <container-name> gitlab-runner <comando>

# Ex.:

docker exec -it <container-name> gitlab-runner start