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/nixos-container-completion.sh

@@ -0,0 +1,33 @@
+#!/usr/bin/env bash
+
+_nixos-container() {
+    local cur prev opts
+    COMPREPLY=()
+    cur="${COMP_WORDS[COMP_CWORD]}"
+    prev="${COMP_WORDS[COMP_CWORD-1]}"
+    opts="list create destroy start stop status update login root-login run show-ip show-host-key"
+    startstop_opts=$(nixos-container list)
+    update_opts="--config"
+
+    if [[ "$prev" == "nixos-container" ]]
+    then
+        COMPREPLY=( $(compgen -W "${opts}" -- ${cur}) )
+        return 0
+    fi
+
+    if [[ $(echo "$opts" | grep "$prev") ]]
+    then
+        if [[ "$prev" == "start" || "$prev" == "stop" ]]
+        then
+            COMPREPLY=( $(compgen -W "${startstop_opts}" -- ${cur}) )
+            return 0
+        elif [[ "$prev" == "update" ]]
+        then
+            COMPREPLY=( $(compgen -W "${update_opts}" -- ${cur}) )
+            return 0
+        fi
+    fi
+}
+
+complete -F _nixos-container nixos-container
+