Allowing to set a value by the user is asking for trouble as only a very few values are actually supported!

I agree, but problem is that sometimes (like today) we have test failures because a new browser image has been pushed, and we need to validate that it's not a regression on our side. To do so we need to execute test with a specific tag for the browser docker container, and right now the only solution is by changing the hardcoded value, which is not very handy. The idea here is to have the capability to change this value only for debugging purpose.