Motivation for this change

Currently the uid of the build user ends up in the image, this makes the permissions deterministic and configurable.

dockerTools.buildImage {
  name = "foo";
  contents = [ pkgs.busybox ];

  uid = 42;
  extraCommands = ''
    touch foo
  '';
}

docker run --rm -it foo /bin/ls -l foo
# -rw-r--r--    1 42        0                0 Jan  1  1970 foo

Things done

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • Linux
• 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 nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.