Running application on subpath

[bandaangosta]

Hello.
I am trying to migrate a web application from using the tiangolo/uwsgi-nginx-flask image to this one, based on Meinheld, Gunicorn and Flask.
My application runs behind a reverse proxy implemented with Traefik, and is mounted on a subpath. For example, on "/demo".
I have a cookiecutter template that easily implements most of this setup, except for the Traefik bits, which I add on deployment using docker-compose and a docker-compose.yml file like this:

version: '3'

services:
  # The reverse proxy service (Træfik)
  reverse-proxy:
    image: traefik  # The official Traefik docker image
    command: --api --docker   # Enables the web UI and tells Træfik to listen to docker
    restart: always
    ports:
      - "80:80"      # The HTTP port
      - "8080:8080"  # The Web UI (enabled by --api)
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock  # So that Traefik can listen to the Docker events

  myapp:
    build:
      context: ./myapp/
      dockerfile: Dockerfile.nginx-uwsgi # just change to Dockerfile.meinheld-gunicorn to use this image as base
    labels:
      - "traefik.frontend.rule=PathPrefix:/demo"
      - "traefik.frontend.priority=1"
    depends_on:
      - reverse-proxy

When using nginx and uwgsi, I need to add a couple of lines to the uwsgi.ini config file, following hints from Flask uwsgi docs:

[uwsgi]
; Uncomment following lines if not mounting the application on root "/" (see http://flask.pocoo.org/docs/1.0/deploying/uwsgi/)
manage-script-name   <--- ADDED FOR MOUNTING ON SUBPATH /demo
mount = /demo=myapp:app   <--- ADDED FOR MOUNTING ON SUBPATH /demo
module = myapp
callable = app
uid = uwsgi
gid = uwsgi

How can I achieve the same thing when using this Meinheld + Gunicorn image?

Gunicorn docs briefly mentions that SCRIPT_NAME can be set in the environment for Gunicorn to pick up. meinheld-gunicorn-flask image does not provide SCRIPT_NAME as an environment variable to set (I tried it anyway), so the next thing I tried is adding this setting in the gunicorn_conf.py file. Following this issue comment for format, I added the following to gunicorn_conf.py (tried with and without the slash):

# Environment variables
raw_env = [
    "SCRIPT_NAME=/demo"
    ]

Not working.
Can you help me with this one? Is this a problem with the image or am I just trying to implement this subpath feature the wrong way here?