Support specifying browser version

[ghostwords]

When run via Docker now, we get the latest stable version of Firefox/Chrome/Edge. While this is good default behavior, we should also be able to specify a specific browser version (Chrome 103.0.5056.0, Firefox 100.0.1, ...), or at least branch (Beta, Dev/Canary, ...).

This probably involves adding a script that figures out the appropriate Firefox/Chrome/WebDriver version (for example), and builds a custom image for the given browser and corresponding driver versions. We will then update runscan.sh to use the new script, and update the Dockerfile to inherit from the generated image (instead of the selenium/standalone-firefox (or -chrome or -edge) image from Docker Hub).