Add nginx reverse proxy documentation for S3 API

Fixes #7407

Add comprehensive documentation and example configuration for using
nginx as a reverse proxy with SeaweedFS S3 API while maintaining AWS
Signature V4 authentication compatibility.

Changes:
- Add docker/nginx/README.md with detailed setup guide
- Add docker/nginx/s3-example.conf with working configuration
- Update docker/nginx/proxy.conf with important S3 notes

The documentation covers:
- Critical requirements for AWS Signature V4 authentication
- Common mistakes and why they break S3 authentication
- Complete working nginx configurations
- Debugging tips and troubleshooting
- Performance tuning recommendations

docker/nginx/proxy.conf

@@ -1,5 +1,13 @@
 # HTTP 1.1 support
 proxy_http_version 1.1;
+
+# IMPORTANT: For S3 API with AWS Signature V4 authentication, you MUST:
+# 1. Use $http_host (not $host) to preserve port numbers
+# 2. Keep proxy_pass_request_headers on (default) to pass all headers unchanged
+# 3. Set proxy_request_buffering off for chunked uploads
+# 4. NOT explicitly set Authorization or X-Amz-* headers
+# See docker/nginx/README.md for detailed S3 reverse proxy configuration
+
 #proxy_buffering off;
 proxy_set_header Host $http_host;
 proxy_set_header Upgrade $http_upgrade;
@@ -20,7 +28,7 @@ proxy_buffers 64 1m;  # buffers used for reading a response from the proxied ser
 proxy_buffer_size 8k; # maximum size of the data that nginx can receive from the server at a time is set
 proxy_busy_buffers_size 2m;
 
-proxy_request_buffering on;  # PUT buffering
+proxy_request_buffering on;  # PUT buffering (set to 'off' for S3 chunked uploads)
 client_body_buffer_size 64m; # buffer size for reading client request body
 client_max_body_size    64m;