By default /etc/passwd in container build sandboxes have two users
(root,nixbld) mapped to root. This confuses nix especially it behaves
different if it runs as root. setuid/setgid() is not enough because ssh
will break if the current uid does not exist in /etc/passwd.
Along with this we now also only run the setup for setting up the
network bridge and cgroup filesystems once and not per container.
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