gitlab runner configuration with docker executor

gitlab-runner-config.toml

concurrent = 2
check_interval = 2

[[runners]]
  name = "gcp-runner"
  url = "https://gitlab.com/"
  token = "xxxxxx"
  executor = "docker"
  [runners.docker]
    tls_verify = false
    image = "docker:latest"
    privileged = true
    disable_cache = false
    volumes = ["/cache:/cache"]
    shm_size = 0
  [runners.cache]