Replace downloaded Selenium server with Docker container

Instead of downloading the Selenium server and requiring a specific
Firefox version to be installed in the system now the Selenium server is
run using one of the official Selenium Docker images, which provides
both the Selenium server and the appropriate version of Firefox.

Moreover, as it is run inside the Docker container, the web browser is
now run in headless mode; however, if needed, it can still be viewed
through VNC.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>