Django Github CI cannot connect to mysql 'db'

Question

Issue

When trying to setup a Github action to test a Django project automatically, we ran into an issue with the django.yml setup. Whenever we ran the .yml, we got this exception on the last step (python manage.py test):

django.db.utils.OperationalError: (2005, "Unknown MySQL server host 'db' (-3)")

To state, our docker environment and tests alone work fine, just when trying to do this in a Github action, we get issues.

What we have tried

1. We have tried running a docker-compose up -d before running the test, to know for sure that our DB is running.
2. Tried adding environment variables with DB info like this: https://github.com/Cuda-Chen/django-mysql-github-actions-demo/blob/main/.github/workflows/django-ci.yml

Current code

name: Django CI

on:
  push:
    branches: [ main ]
  pull_request:
    branches: [ main ]

jobs:
  build:

    runs-on: ubuntu-latest
    strategy:
      max-parallel: 4
      matrix:
        python-version: [3.8]

    steps:
    - uses: actions/checkout@v3
    - name: Set up Python ${{ matrix.python-version }}
      uses: actions/setup-python@v3
      with:
        python-version: ${{ matrix.python-version }}
    - name: Install Dependencies
      run: |
        python -m pip install --upgrade pip
        pip install -r requirements.txt
    - name: Run Tests
      run: |
        python manage.py test

Does anyone know what is going on here, and how we can resolve this?