feat(nginx-proxy): Implement ssl_reject_handshake for missing certificates

Instead of serving 500 errors with default certificates when a vhost
has no valid SSL certificate, nginx now:

1. Uses default certificate (if available) and returns 503
2. Rejects the SSL/TLS handshake entirely (if no default cert)

Benefits:
- Prevents certificate warning dialogs in browsers
- More secure - doesn't expose invalid/default certificates
- Cleaner failure mode for unknown hosts
- Matches upstream jwilder/nginx-proxy behavior

Changes:
- Updated fallback HTTPS server block to use ssl_reject_handshake
- Added condition to check for default certificate availability
- Changed status code from 500 to 503 for better HTTP semantics
- Documented SSL certificate handling behavior in README

Author: mrrobot47

nginx-proxy/README.md

@@ -130,6 +130,28 @@ deny all;
 
 ---
 
+## SSL Certificate Handling
+
+### Certificate Lookup
+
+The proxy automatically detects SSL certificates from `/etc/nginx/certs/`:
+
+```bash
+/etc/nginx/certs/example.com.crt
+/etc/nginx/certs/example.com.key
+```
+
+### Fallback Certificate Behavior
+
+When a vhost is accessed via HTTPS but no matching certificate is found:
+
+1. **If default certificate exists:** Uses `/etc/nginx/certs/default.crt` and returns 503
+2. **If no default certificate:** Rejects the SSL/TLS handshake
+
+This prevents certificate warning dialogs in browsers and improves security by not exposing invalid certificates.
+
+---
+
 ## Environment Variables
 
 | Variable | Description | Default |

nginx-proxy/nginx.tmpl

@@ -554,7 +554,7 @@ server {
 	{{ end }}
 }
 
-{{ if (and (not $is_https) (exists "/etc/nginx/certs/default.crt") (exists "/etc/nginx/certs/default.key")) }}
+{{ if (and (not $is_https) (ne $https_method "nohttps")) }}
 server {
 	server_name {{ $host }};
 	listen 443 ssl {{ $default_server }};
@@ -563,10 +563,17 @@ server {
 	listen [::]:443 ssl {{ $default_server }};
 	http2 on;
 	{{ end }}
-	return 500;
 
+	{{ if (and (exists "/etc/nginx/certs/default.crt") (exists "/etc/nginx/certs/default.key")) }}
+	# Use default certificate as fallback
 	ssl_certificate /etc/nginx/certs/default.crt;
 	ssl_certificate_key /etc/nginx/certs/default.key;
+	{{ else }}
+	# No valid certificate for this vhost nor default certificate found, so reject SSL handshake
+	ssl_reject_handshake on;
+	{{ end }}
+
+	return 503;
 }
 {{ end }}