Is your feature request related to a problem? Please describe.
Our fastapi service is place behind a proxy using path-based routing - i.e. /docs becames /foo/docs

This result in the default swagger spec as well as the swagger UI to fails as the openapi path passed to the html is an absolute ref (i.e. /docs or /prefix/docs) which will resolve incorrectly if there is a non-fastapi path routing.

See https://github.com/tiangolo/fastapi/blob/fc7d123347591c0d79505ee0c6a3a5ace70192f1/fastapi/applications.py#L91

Describe the solution you'd like
Either incorporate a base href argument to fast API, or set the openapi_url to relative ref (i.e. ./docs or ./prefix/docs).

Similar fix should be done for the openapi spec generation.

I would prefer the former solution as it will also correctly generate the openapi spec (to include the non-fastapi prefix).

Describe alternatives you've considered
Currently I provision my own custom route with the modified swagger UI HTML as well as the openapi spec.