feat: add tiktoken cache directory support for offline deployment

- Mount tiktoken cache volume in docker-compose
- Add TIKTOKEN_CACHE_DIR environment variable
- Update env.example with tiktoken cache configuration
- Added /data to gitignore
- Added tiktoken env example

.gitignore

@@ -50,6 +50,7 @@ rag_storage/
 examples/input/
 examples/output/
 output*/
+data/
 
 # Miscellaneous
 .DS_Store