Containers: Use /etc/resolv.conf supplied by the host

This used to work with systemd-nspawn 203, because it bind-mounted
/etc/resolv.conf (so openresolv couldn't overwrite it). Now it's just
copied, so we need some special handling.

nixos/modules/system/boot/stage-2-init.sh

@@ -96,8 +96,6 @@ mkdir -m 0755 -p /etc/nixos
 
 # Miscellaneous boot time cleanup.
 rm -rf /var/run /var/lock
-rm -f /etc/resolv.conf
-touch /etc/resolv.conf
 rm -f /etc/{group,passwd,shadow}.lock
 
 if test -n "@cleanTmpDir@"; then
@@ -149,6 +147,12 @@ if test -n "$resumeDevice"; then
 fi
 
 
+# Use /etc/resolv.conf supplied by systemd-nspawn, if applicable.
+if [ -n "@useHostResolvConf@" -a -e /etc/resolv.conf ]; then
+    cat /etc/resolv.conf | resolvconf -m 1000 -a host
+fi
+
+
 # Run the script that performs all configuration activation that does
 # not have to be done at boot time.
 echo "running activation script..."

nixos/modules/system/boot/stage-2.nix

@@ -19,11 +19,13 @@ let
     isExecutable = true;
     inherit (config.boot) devShmSize runSize cleanTmpDir;
     inherit (config.nix) readOnlyStore;
+    inherit (config.networking) useHostResolvConf;
     ttyGid = config.ids.gids.tty;
     path =
       [ pkgs.coreutils
         pkgs.utillinux
         pkgs.sysvtools
+        pkgs.openresolv
       ] ++ (optional config.boot.cleanTmpDir pkgs.findutils)
       ++ optional config.nix.readOnlyStore readonlyMountpoint;
     postBootCommands = pkgs.writeText "local-cmds"