Motivation for this change

The system output usually contains a symlink from /etc to the static
configuration for the benefit of the stage-1 script in the initrd. The stage-2
script is usually started in the real root without such a symlink. In a
container, there is no stage-1 and the system output is used directly as a real
root. If the symlink is present, setup-etc.pl will create a symlink cycle and
the system cannot boot. There is no reason for the /etc link to exist in a
container because setup-etc.pl will create the necessary files. The container
module will now remove the /etc symlink and create an empty directory. The empty
/etc is for container managers to populate it with site-specific settings; for
example, to set the hostname. This is required to boot NixOS in an LXC container
on another host.

See also: #9735

@grahamc I asked you to review this because I expect you will have opinions about running NixOS in a container. 😄 However, this module has not seen a lot of love recently (or ever) so I can't tell from the Git history who is qualified. If you feel someone is more qualified, please refer me.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.