def format_container_name(name, special_characters=None):
if special_characters is None:<EOL><INDENT>special_characters = []<EOL><DEDENT>return '<STR_LIT>'.join(e.lower()<EOL>for e in name if e.isalnum() or e in special_characters)<EOL>
format_container_name will take a name supplied by the user,
    remove all special characters (except for those defined by "special-characters"
    and return the new image name.