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

package.json

@@ -1,7 +1,7 @@
 {
   "name": "gitea-mirror",
   "type": "module",
-  "version": "2.20.1",
+  "version": "2.21.0",
   "engines": {
     "bun": ">=1.2.9"
   },