Provide an updated arc wrapper to build by-user images
to be able to run comfortably Arcanist or a shell with
it as an unprivileged user.

As this could be considered an hacky solution, or as it
could be used on machine where docker build isn't
available, we keep the legacy one too.

Document both the wrappers.

This change is imported from Nasqueron Operations repository,
sync'ed at commit rOPScfb07164325c.

Both wrappers have been tested on Dwellers,
as the arc command.