Add a default nginx proxy cache location

In order to use nginx proxy caches a named zone must be created via `proxy_cache_path` however I believe that can only be called within the HTTP context. This means there's no way to use proxy caches if the `-C` server context configs are used.

I think it should be fine to define a cache zone in the default config that can then be optionally used in the app specific configs which would give us more flexibility.