nixos-containers: init package (#16959)

This moves nixos-containers into its own package so that it can be
relied upon by other packages/systems. This should make development
using dynamic containers much easier.

pkgs/tools/virtualization/nixos-container/default.nix

@@ -0,0 +1,17 @@
+{ substituteAll, perl, perlPackages, shadow, utillinux }:
+
+substituteAll {
+    name = "nixos-container";
+    dir = "bin";
+    isExecutable = true;
+    src = ./nixos-container.pl;
+    perl = "${perl}/bin/perl -I${perlPackages.FileSlurp}/lib/perl5/site_perl";
+    su = "${shadow.su}/bin/su";
+    inherit utillinux;
+
+    postInstall = ''
+      t=$out/etc/bash_completion.d
+      mkdir -p $t
+      cp ${./nixos-container-completion.sh} $t/nixos-container
+    '';
+}