By default, Nginx does proxy buffering and it may store big replies to a
temporary file up to 1 GB. It also means that Nginx will read data as
fast as possible from Garage, even if the client downloads slowly. Both
behaviours are often not wanted, so disable this temporary file in the example.
Ref: https://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_buffering
Also add an example of upstream with a "backup" server, which may be
useful to only use remote servers as fallback.
