Finally figured it out myself. What was happening was I was using the wrong URL for the selenium hub. I also then ran on to the problem of the selenium service unable to access the php:7.4-apache server (localhost from the selenium server refers to itself, duh). The relevant changes I made were:

• changed `http://selenium:4444’ to ‘http://selenium:4444/wd/hub’
• added options: --network-alias=testserver under test -> container so that selenium container can access it through ‘http://testserver’

The relevant section on the workflow has been changed to this:

  test:
    needs: build
    runs-on: ubuntu-latest
    container: 
      image: php:7.4-apache
      ports: 
        - 80:80
      options: --network-alias=testserver 
env:
  TEST_BASE_URL: http://testserver
  SELENIUM_HOST: http://selenium:4444/wd/hub

services:
  selenium:
    image: selenium/standalone-chrome
    # options: --health-cmd "curl http://selenium:4444" --health-interval=5s
    ports: 
      - 4444:4444
steps:
- name: Download built files
  uses: actions/download-artifact@v2
  with:
    name: www-build

- name: Extract artifacts
  run: |
    tar -xzf www-build.tar.gz
    ls -l

- name: Copy files to web root
  run: |
    cp -R ./www/* /var/www/html

- name: Restart apache server
  run: service apache2 restart

- name: Check if site is accessible
  run: |
    curl $TEST_BASE_URL

- name: Run Test
  run: ./vendor/bin/phpunit test