Docker compose Flask container throws error during start-up (exec /usr/local/bin/gunicorn: exec format error)

I want to deploy a service contains a flask, Nginx and Postgres db. Upon creating a cluster in ECS, I get the error from the flask container exec /usr/local/bin/gunicorn: exec format error. Dockerfile

# pull official base image
FROM python:3.9.6-alpine

# Update pip
RUN python -m pip install --upgrade pip

# Expose port
EXPOSE 8080

# set work directory
WORKDIR /app

# Prevents Python from writing pyc files to disc (equivalent to python -B option)
ENV PYTHONDONTWRITEBYTECODE=1

# Prevents Python from buffering stdout and stderr (equivalent to python -u option)
ENV PYTHONUNBUFFERED=1

RUN ls -ltr /app
# install dependencies
RUN pip install --upgrade pip
COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt
RUN pip install gunicorn

COPY . /app

# Creates a non-root user with an explicit UID and adds permission to access the /app folder
# For more info, please refer to https://aka.ms/vscode-docker-python-configure-containers
RUN adduser -u 5678 --disabled-password --gecos "" appuser && chown -R appuser /app
USER appuser


ENV FLASK_RUN_PORT 8080
ENV FLASK_APP app.py
CMD ["/usr/local/bin/gunicorn", "--config", "gunicorn_config.py", "app:app"]

Docker compose file

x-aws-vpc: "vpc-063f8b21a548c037c"
#x-aws-cluster: "Flask-API-Cluster"

services:
  flask_api:
    container_name: flaskapi
    image: dockerelvis/flask:latest
    logging:
      driver: awslogs
      options:
        awslogs-region: eu-west-2
    restart: unless-stopped
    volumes:
      - flask-data:/app
    networks:
      - flask-network
      - db-network
    depends_on:
      - database
    links:
      - database
    env_file:
      - backend/.env
    healthcheck:
      test: wget --no-verbose --tries=1 --spider http://localhost || exit 1
      interval: 60s
      retries: 5
      start_period: 20s
      timeout: 10s

  database:
    container_name: postgresdb
    image: postgres:latest
    restart: unless-stopped
    env_file:
      - backend/.env
    networks:
      - db-network
    volumes:
      - postgres-data:/var/lib/postgresql/data

  nginx:
    image: dockerelvis/nginx:latest
    restart: always
    volumes:
      - static_volume:/home/app/web/staticfiles
    ports:
      - target: 9000
        x-aws-protocol: http
    networks:
      - flask-network
    depends_on:
      - flask_api

volumes:
  postgres-data:
    external: false
    name: postgres-data
  static_volume:
    external: false
    name: static_volume
  flask-data:
    external: false
    name: flask-data

networks:
  flask-network:
    external: false
    name: flask-network
  db-network:
    external: false
    name: db-network

x-aws-logs_retention: 10