Docker executor for running tests optimization

I have setup my CI to use a base python docker image to run unit tests.

test:
    image: registry/btg/docker-images/linux/dev:latest
    services:
        - postgres:10.5
        - redis:latest
    stage: test
    script:
        - "pwd"
        - "pip3 install -U pip"
        - "pip3 install -r requirements_test.txt -c constraints.txt"
        - "python3 manage.py test"

The above is working fine but slow, as it installs requirements everytime even there is no requirement changes.

I am aware I can prepare another base docker image with dockerfile using the ADD instruction to optimize as per Speed Up The Build of Your Python Docker Image ยท vsupalov.com however the docker executor does not involve docker build.

How do I optimize this step?

If you need to prepare your own image you can do it using DinD or Kaniko