Unable to start valhalla-scripted with custom_files set as read-only

I am using the Docker image and I've recently updated from version `3.3.0` to `3.6.0` and tying `custom_files` to a read-only file system does not appear to be working. I am getting the follow error in the logs:

```
INFO: Found routing tiles with 'use_tiles_ignore_pbf' True.
INFO: Jumping directly to the tile loading!
error opening output file: Read-only file system
```

Looking through the Docker script and code a bit, it appears that setting `update_existing_config` to `False` is needed to prevent doing anything to `valhalla.json` on startup, however I believe something in `valhalla/config` or `rapidjson::read_json` wants to open this file in read/write mode.

If there is a setting I am missing then I am all ears. Thanks!