feat: add custom CA certificate support

- Add support for custom CA certificates in Docker setup
- Two mounting options: individual certs or system CA bundle
- Automatic detection and configuration via NODE_EXTRA_CA_CERTS
- Enhanced documentation with setup guide in certs/README.md
- Added ca-certificates package to Alpine base image
- Updated docker-compose with clear volume mount examples
- Bump version to 2.21.0

.env.example

@@ -37,3 +37,10 @@ JWT_SECRET=change-this-to-a-secure-random-string-in-production
 # Users can configure cleanup settings through the web interface
 # CLEANUP_ENABLED=false
 # CLEANUP_RETENTION_DAYS=7
+
+# Optional TLS/SSL Configuration
+# Option 1: Mount custom CA certificates in ./certs directory as .crt files
+#          The container will automatically combine them into a CA bundle
+# Option 2: Mount your system CA bundle at /etc/ssl/certs/ca-certificates.crt
+#          See docker-compose.yml for volume mount examples
+# GITEA_SKIP_TLS_VERIFY=false  # WARNING: Only use for testing, disables TLS verification