example for unshare from the shell / running nix commands in a /nix directory not at root

nix-container-run.sh

#!/usr/bin/env nix-shell
#! nix-shell -i sh -p nix

set -e

ROOT="$1"; shift

USER_MOUNTS=$(nix-build --no-out-link shell.nix -A user-mounts)
ZSH=$(nix-build --no-out-link "<nixpkgs>" -A zsh)
CU=$(nix-build --no-out-link "<nixpkgs>" -A coreutils)
UL=$(nix-build --no-out-link "<nixpkgs>" -A utillinux)
NIX=$(nix-build --no-out-link "<nixpkgs>" -A nix)
SUDO=$(nix-build --no-out-link "<nixpkgs>" -A sudo)

nix copy --no-check-sigs --to local?root=$ROOT $USER_MOUNTS $ZSH $NIX $CU $SUDO $UL

exec $USER_MOUNTS/bin/unshare-user-mounts "
mount --make-rprivate /
mount --rbind $ROOT/nix /nix
export PATH='$ZSH/bin:$NIX/bin:$CU/bin:$SUDO/bin:$UL/bin'
" "$@"

revertuid.c

#define _GNU_SOURCE
#include <sched.h>

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

#include <unistd.h>
#include <string.h>

void remap(const char *conf_path, const char *id) {
  int fd;
  fd=open(conf_path, O_WRONLY);
  write(fd,id,strlen(id));
  write(fd," 0 1",4);
  close(fd);
}

int main(int argc, char *argv[]) {
    int fd;

    unshare(CLONE_NEWUSER);
    fd=open("/proc/self/setgroups",O_WRONLY);
    write(fd,"deny",4);
    close(fd);
    remap("/proc/self/uid_map", argv[1]);
    remap("/proc/self/gid_map", argv[2]);
    execvp(argv[3],argv+3);
}

shell.nix

with import <nixpkgs> {};

{

  user-mounts = callPackage ./user-mounts.nix {};
  
}

unshare-user-mounts.in

#!/bin/sh

set -evx

SETUP="$1"; shift
U=$(id -u)
G=$(id -g)
exec unshare -rmU -- sh -evxc "
$SETUP
exec @out@/bin/revertuid $U $G $@
"

user-mounts.nix

{ runCommand, gcc }:

runCommand "user-mount-tools" {
  nativeBuildInputs = [
    gcc
  ];
} ''
  mkdir -p $out/bin
  gcc -o $out/bin/revertuid ${./revertuid.c}
  substituteAll ${./unshare-user-mounts.in} $out/bin/unshare-user-mounts
  chmod +x $out/bin/unshare-user-mounts
  fixupPhase
''

revertuid.c is necessary, because setgroups gid_maps and uid_maps must be written together in a way that can't be done from shell

This looks closely related to a question I recently asked on Reddit - I'd be curious if you have any advice on this. It seems unshare is probably the/a key I'm missing.