DaskHub cannot be placed in a subdirectory URL

**Describe the issue**:

I installed the vanilla DaskHub Helm chart, and routed traffic via an NGINX reverse proxy with a dedicated location for the JupyterHub service, e.g. `https:///jupyterhub/`.

This meant overriding that part of the `values.yaml` as follows:
```
jupyterhub:
hub:
baseUrl: /jupyterhub/
```

The JupyterHub service then all works as expected at that location. However, when calling the Dask Gateway service (which is configured as a service of JupyterHub via the custom configuration here: https://github.com/dask/helm-chart/blob/main/daskhub/values.yaml#L27), the service is not found.

Inspecting the JupyterHub proxy's logs we see (including previous line as reference):
```
09:08:05.551 [ConfigProxy] info: 201 POST /api/routes/jupyterhub/user/user1
09:08:16.923 [ConfigProxy] error: 404 POST /services/dask-gateway/api/v1/clusters/
```

I tried modifying the `extraConfig` passed to JupyterHub to manually specify the subdirectory in the URL to no avail.

It could be that this is a broader issue which should be raised in the JupyterHub project - let me know if I should do this.

**Environment**:

- DaskHub Helm version: daskhub-2023.1.0