"Plugin caching_sha2_password could not be loaded" references mariadb file from mysql service

Why not try it with an other image?

I used:

default:
  image: ubuntu:20.04
    services:
      - mysql:8.0

and

  before_script:
    - apt -y update
    - apt -y install apt-utils
    - apt -y install net-tools python3.8 python3-pip mysql-client libmysqlclient-dev
    - apt -y upgrade
    - pip3 install -r requirements.txt

Worked for a Django project. Of course you use the image and python version that comes closes to your production environment.