Support subdirectory root paths when API is behind a proxy

[alyssadai]

Currently, even when a reverse proxy is set up to strip a path prefix, the interactive docs /docs path won't work because the docs UI requires the OpenAPI schema to declare what the proxied server URL is (see https://fastapi.tiangolo.com/advanced/behind-a-proxy/#proxy-with-a-stripped-path-prefix).

Steps to implement

• Add environment variable for the root path
• Set the root_path parameter when initializing FastAPI app https://fastapi.tiangolo.com/advanced/behind-a-proxy/#setting-the-root_path-in-the-fastapi-app