使用gitlab CI在Firefox上运行Selenium

Question

我正在尝试使用gitlab使用selenium / firefox运行pytest代码，我得到了

 :Exceptions:
         - WebDriverException : Raised either when it can't start the service
           or when it can't connect to the service

.gitlab-ci.yml文件内部：

Test:
  image: python:3.7
  stage: Test
  script:
    - echo "testing"
    - pip install selenium
    - pip install pytest
    - pip install webdriver
    - pytest sample.py
  services:
    - name: selenium/standalone-firefox
      alias: selenium

我应该改变什么？