Python flask application access to docker secrets in a swarm

Question

I'm new to Docker and want to deploy a flask+gunicorn project. So far, I've got the following Dockerfile.

# Get official base image
FROM python:3.7-slim-buster

# Setting my working directory
RUN mkdir -p /usr/src/app
WORKDIR /usr/src/app

# Setting my environment variables
ENV REDIS_HOST [...omit here...]
ENV REDIS_PORT [...omit here...]
ENV REDIS_DB_WHITELIST [...omit here...]
ENV MYSQL_HOST [...omit here...]
ENV MYSQL_PORT [...omit here...]
ENV MYSQL_DB_DUMMY [...omit here...]

# Copy project for later
COPY . /usr/src/app/

# Install dependencies
RUN pip install --upgrade pip
RUN pip install -r requirements.txt
RUN pip install gunicorn

EXPOSE 5000

RUN chmod +x ./entrypoint.sh
ENTRYPOINT ["sh", "entrypoint.sh"]

My docker-compose.yml has the following:

version: "3.9"

secrets:
  FLASK_SECRET_KEY:
    external: true
  MYSQL_USER:
    external: true
  MYSQL_PASSWORD:
    external: true

services:
  web:
    image: flask-app:v0.1.0
    environment:
      FLASK_SECRET_KEY_FILE: /run/secrets/FLASK_SECRET_KEY
      MYSQL_USER_FILE: /run/secrets/MYSQL_USER
      MYSQL_PASSWORD_FILE: /run/secrets/MYSQL_PASSWORD
    ports:
      - "5000:5000"
    secrets:
      - FLASK_SECRET_KEY
      - MYSQL_USER
      - MYSQL_PASSWORD

After doing some research, it appears that using the docker stack deploy —compose-file=docker-compose.yml flask-app command is the only way to get docker secrets. Obviously, I have three sensitive data to store in Docker secrets: FLASK SECRET KEY, MYSQL USER, and MYSQL PASSWORD. It turns out that the programme is failing to execute, and I'm guessing that mysql user = os.environ['MYSQL USER'] etc. in the python script is failing to access the environment variable.

I'm not sure how to get sensitive info from Docker secrets via Dockerfile or docker-compose.yml, and I'd appreciate it if someone could correct me if I'm incorrect.