reuse pooled upstream connections for nginx auth subrequests

The /auth subrequest proxied to a variable address, which nginx can't pool, and its location-level headers dropped the inherited Connection header so nginx sent Connection: close. Every authenticated request opened and closed its own loopback connection to the API. It now proxies to the frigate_api upstream with an empty Connection header so the keepalive pool is reused.

docker/main/rootfs/usr/local/nginx/conf/auth_location.conf

@@ -1,10 +1,8 @@
-set $upstream_auth http://127.0.0.1:5001/auth;
-
 ## Virtual endpoint created by nginx to forward auth requests.
 location /auth {
     ## Essential Proxy Configuration
     internal;
-    proxy_pass $upstream_auth;
+    proxy_pass http://frigate_api/auth;
 
     ## Headers
 
@@ -39,6 +37,7 @@ location /auth {
     proxy_next_upstream error timeout invalid_header http_500 http_502 http_503; # Timeout if the real server is dead
     proxy_redirect http:// $scheme://;
     proxy_http_version 1.1;
+    proxy_set_header Connection "";
     proxy_cache_bypass $cookie_session;
     proxy_no_cache $cookie_session;
     proxy_buffers 4 32k;