Change Details

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.