dockerTools.examples: fix the nginx image

Nginx now requires directories `/var/log/nginx` and `/var/cache/nginx`
to be existing.

The objective is to fix the test `nixosTests.docker-tools`.

pkgs/build-support/docker/examples.nix

@@ -69,6 +69,12 @@ rec {
     tag = "latest";
     contents = pkgs.nginx;
 
+    extraCommands = ''
+      # nginx still tries to read this directory even if error_log
+      # directive is specifying another file :/
+      mkdir -p var/log/nginx
+      mkdir -p var/cache/nginx
+    '';
     runAsRoot = ''
       #!${pkgs.stdenv.shell}
       ${shadowSetup}