Gitlab Runner:无法连接到Docker后台程序

当我试图构建docker映像时,出现以下错误消息:

无法连接到位于tcp://127.0.0.1:2375的Docker守护程序。是 docker守护进程正在运行?

我的gitlab ci文件如下:

image: docker:19

stages:
    - build

variables:
    DOCKER_HOST: tcp://127.0.0.1:2375
    DOCKER_TLS_CERTDIR: ""

before_script:
    - docker login -u "$CI_REGISTRY_USER" -p "$CI_REGISTRY_PASSWORD" $CI_REGISTRY

first-job:
    stage: build
    script:
        - docker build -t "$CI_REGISTRY_IMAGE/web:latest" .

我的跑步者配置如下:

[[runners]]
  name = "sample_runner"
  url = "https://gitlab.com/"
  token = "XXXXXXXXXX"
  executor = "docker"
  [runners.custom_build_dir]
  [runners.docker]
    tls_verify = false
    image = "docker:19"
    privileged = false
    disable_entrypoint_overwrite = false
    oom_kill_disable = false
    disable_cache = false
    volumes = ["/cache"]
    shm_size = 0
  [runners.cache]
    [runners.cache.s3]
    [runners.cache.gcs]

docker login命令运行良好,但在构建时失败。有什么想法吗?