Support specifying browser version

When run via Docker now, we get the [latest stable version of Firefox/Chrome/Edge](https://github.com/EFForg/badger-sett/blob/8306b61e278fdc8d34a0facb04d5501118e368cb/Dockerfile#L4). While this is good default behavior, we should also be able to specify a specific browser version ([Chrome `103.0.5056.0`](https://chromiumdash.appspot.com/releases?platform=Linux), [Firefox `100.0.1`](https://ftp.mozilla.org/pub/firefox/releases/100.0.1/linux-x86_64/en-US/), ...), or at least branch (Beta, Dev/Canary, ...).

This probably involves adding a script that figures out the appropriate Firefox/Chrome/WebDriver version ([for example](https://github.com/EFForg/privacybadger/blob/d4555319ad59b65b42e21142016c556a668a0f35/scripts/setup_travis.sh#L82-L96)), and [builds a custom image](https://github.com/SeleniumHQ/docker-selenium/wiki/Building-your-own-images) for the given browser and corresponding driver versions. We will then update [runscan.sh](https://github.com/EFForg/badger-sett/blob/master/runscan.sh) to use the new script, and update the [Dockerfile](https://github.com/EFForg/badger-sett/blob/master/Dockerfile) to inherit from the generated image (instead of the [`selenium/standalone-firefox`](https://hub.docker.com/r/selenium/standalone-firefox) (or `-chrome` or `-edge`) image from Docker Hub).