Use latest Firefox version with Selenium 2 on GitHub Actions

[aik099]

The https://hub.docker.com/r/selenium/standalone-firefox image provided by Selenium only supports Firefox 47 when used with Selenium server 2.x versions. This causes problems in a test suite because a working code (e.g. dragging a div into itself in #359) fails due:

• either outdated Firefox version
• either outdated geckodriver version

Need to create an image with a modern Firefox/geckodriver version and use it on GitHub Actions.