nixos/containers: Create an empty machine-id file

Since systemd version 230, it is required to have a machine-id file
prior to the startup of the container. If the file is empty, a transient
machine ID is generated by systemd-nspawn.

See systemd/systemd#3014 for more details on the matter.

This unbreaks all of the containers-* NixOS tests.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @edolstra
Closes: #15808

nixos/modules/virtualisation/containers.nix

@@ -309,6 +309,10 @@ in
             touch "$root/etc/os-release"
           fi
 
+          if ! [ -e "$root/etc/machine-id" ]; then
+            touch "$root/etc/machine-id"
+          fi
+
           mkdir -p -m 0755 \
             "/nix/var/nix/profiles/per-container/$INSTANCE" \
             "/nix/var/nix/gcroots/per-container/$INSTANCE"