Related to #7207
Modify the code to allow the docker container to start in an offline environment for versions >= 0.4.0.
* **backend/open_webui/retrieval/utils.py**
- Import `OFFLINE_MODE` from `open_webui.env`.
- Set `local_files_only` to `True` when `OFFLINE_MODE` is enabled in `snapshot_kwargs`.
* **backend/open_webui/env.py**
- Add logic to set `HF_HUB_OFFLINE` environment variable to `1` when `OFFLINE_MODE` is enabled.
* **README.md**
- Document setting `HF_HUB_OFFLINE` environment variable to `1` for offline environments.
