Update docs with what we've learned from #17986

See #17986

docker/conf-workers/nginx.conf.j2

@@ -38,6 +38,9 @@ server {
 {% if using_unix_sockets %}
         proxy_pass http://unix:/run/main_public.sock;
 {% else %}
+        # note: do not add a path (even a single /) after the port in `proxy_pass`,
+        # otherwise nginx will canonicalise the URI and cause signature verification
+        # errors.
         proxy_pass http://localhost:8080;
 {% endif %}
         proxy_set_header X-Forwarded-For $remote_addr;

docs/reverse_proxy.md

@@ -74,7 +74,7 @@ server {
         proxy_pass http://localhost:8008;
         proxy_set_header X-Forwarded-For $remote_addr;
         proxy_set_header X-Forwarded-Proto $scheme;
-        proxy_set_header Host $host;
+        proxy_set_header Host $host:$server_port;
 
         # Nginx by default only allows file uploads up to 1M in size
         # Increase client_max_body_size to match max_upload_size defined in homeserver.yaml