Add ROOT_PATH env variable to simplify running Titiler behind a proxy

[tayden]

I'd really like to get Titiler running on an existing server, and have requests to the root path /titiler passed to a running docker container running on, e.g. port 9000. However, there's currently no way to specify the root_path parameter in the FastAPI instantiation, so the app has been required to run at the root location /. (see FastAPI docs on running behind a proxy)

This PR allows users to set the root_path parameter of the FastAPI instance via an environment variable so that it's dead simple to run the service behind a reverse proxy. It defaults that root_path location to "/", so nothing anybody currently has running breaks.

I suspect that for this to be accepted, some documentation has to be written up for this behaviour. I'm just not sure where that should go. If someone wants to tell me where to put that, I can fill in those details.

[vincentsarago]

@tayden thanks for the PR

Would it make more sense to add this to https://github.com/developmentseed/titiler/blob/master/src/titiler/application/titiler/application/settings.py#L6 ?

also looking at the CI, the tests fails because this adds a / to the base path, so I guess the default root_path should be ""

[tayden]

@vincentsarago That makes sense to me. I've updated/fixed the code

[vincentsarago]

excellent, thanks @tayden

Tests are failing but it's unrelated to the changes (and have been fixed on the master branch just now). Could you merge from master and add a note in the changelog 🙏

[vincentsarago]

🙏 thanks @tayden