Use argparse for argument parsing rather than reading from
sys.argv directly.
Print multi-line error messages to sys.stderr rather than raise
exceptions. Since the goal is for the user to read them, rather
than a system, this is cleaner.
Use any() to check for existence of
non-dockerfile files, rather than a list comprehension. This
expresses intent better, and is more performant.
Put everything into a function
Add correct #! to tell linux what interpreter to use to execute
this file