Add an optional default value to dock$ARG

[antoine-sachet]

I just added a default argument to add_arg. Currently, this argument is the third (after ahead).

I would prefer to put it in the second spot so you could do dock$ARG("myarg", "myval"), but that would break code that currently does dock$ARG("myarg", TRUE) instead of dock$ARG("myarg", ahead = TRUE).

I also roxygenised the doc. In the new version, R6 objects are finally documented so ?dockerfiler::Dockerfile finally has a whole detailed page. :+1: