Validate builders before running

[eons44]

It should be possible to check all arguments required for all build steps against those provided before executing the first build step.
This would need to be done in a function so that builders which change the next builder (e.g. proxy or in_container) can dynamically validate the args of their downstream targets.
Because builders may change environment variables or pass additional arguments to their children, a validation process like this can never be guaranteed 100% to work. However, failing faster and giving the user information on what to fix would be a dramatic improvement over the current state of "run it and find out!".