add option to disable request buffering.

If push is allowed, buffering requests can cause issues on slow upstreams.
The put requests pile up, and since `proxy_ignore_client_abort` is on...
...the push fails.

If you have trouble pushing, set `PROXY_REQUEST_BUFFERING`to false, then fix
remainig issues/timeouts.

Default is true to not change default behavior.

nginx.conf

@@ -227,6 +227,9 @@ echo "Docker configured with HTTPS_PROXY=$scheme://$http_host/"
 
         proxy_read_timeout 900;
 
+        # Request buffering
+        include /etc/nginx/proxy.request.buffering.conf;
+
         # Use cache locking, with a huge timeout, so that multiple Docker clients asking for the same blob at the same time
         # will wait for the first to finish instead of doing multiple upstream requests.
         proxy_cache_lock on;