dockerTools: fix build failure when building in vm

Fixes build failure for nixos.tests.docker-tools.
The tar command below the changed line relied on the $out directory
being created but it is not being created after the commit below.
97ed6b4565e76286062e6942517a71ae4c9cac72

pkgs/build-support/docker/default.nix

@@ -392,6 +392,7 @@ rec {
 
       postMount = ''
         echo "Packing raw image..."
+        mkdir -p $out
         tar -C mnt --hard-dereference --sort=name --mtime="@$SOURCE_DATE_EPOCH" -cf $out/layer.tar .
       '';