I'm no NGINX or security expert, but this seems to work.

    location /passwd {
        root /etc;
        try_files $uri $uri/ passwd;
    }

The FAQ says that it's probably insecure and to please be gentle :)

This is a very neat idea. I can picture for all sorts of software with a drop-down at the top to select what you want to play with.

Regarding security: From the gist it looks like you're only using network namespaces, when you should probably use an entire container with limits for memory and CPU and isolation for the filesystem as well.

I love this idea so much. It'll definitely come in handy next time I'm writing a config, thanks for sharing!