The last day or two I've been using some of my free time to try to find whey pflask was giving me errors related to group and user mappings on puppy linux (see post). In this search I found some related info about the Linux Kernel Development (from lwn.net) that other people might find interesting. The articles are:
December 5, 2012 Stepping closer to practical containers: "syslog" namespaces
December 13, 2012 User namespaces progress
February 27, 2013 Namespaces in operation, part 5: User namespaces
December 17, 2014 User namespaces and setgroups()
February 17, 2020 Filesystem UID mapping for user namespaces: yet another shiftfs
Also related, here is a general overview of linux namespaces:
https://subscription.packtpub.com/book/ ... nd-cgroups