fix: check trusted proxy login against nginx's peer, not nginx (#1625)

The backend only sees the bundled nginx on loopback, so the allowlist
could only work by trusting 127.0.0.1, which let any client that reached
nginx log in as anyone by sending the identity headers. nginx now passes
its own TCP peer (before real_ip rewriting) in X-Termix-Proxy-Peer, and
loopback connections are judged by that address instead.

docker/nginx-https.conf

@@ -200,6 +200,9 @@ http {
             proxy_http_version 1.1;
             proxy_set_header Host $http_host;
             proxy_set_header X-Real-IP $remote_addr;
+            # The TCP peer before set_real_ip_from rewrites $remote_addr from
+            # X-Forwarded-For; trusted proxy login checks this address.
+            proxy_set_header X-Termix-Proxy-Peer $realip_remote_addr;
             proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
             proxy_set_header X-Forwarded-Proto $proxy_x_forwarded_proto;
             proxy_set_header X-Forwarded-Port $proxy_x_forwarded_port;