How to configure the dependabot.yml for updating whenever a new python patch version for alpine is available on docker hub?

I'm using Docker for codebase and dependabot on GitHub.

What I want

Whenever there's a new patch version for python or new minor version of alpine for the python-alpine image on docker hub see https://hub.docker.com/_/python, I want dependabot to create a new PR for it.

What I tried

This is my dependabot.yml

version: 2
updates:
  - package-ecosystem: "docker" # See documentation for possible values
    directory: "/compose/local/django" # Location of package manifests
    schedule:
      interval: "monthly"

These are the locations of my Dockerfile.

1. for dev: /compose/local/django
2. for production: /compose/production/platform/doap

My Dockerfile at both locations start like this:

FROM python:3.8.7-alpine3.12

ENV PYTHONDONTWRITEBYTECODE 1
ENV PYTHONUNBUFFERED 1

RUN apk update \

I don't think it works. I don't see any PR as the latest is python:3.8.8-alpine3.12. How do I change my dependabot.yml?