Setting up gitlab CI with simple django project

I have a very simple django project with a few unit tests on it and I would like to set up Gitlab to run those tests evry time I make a new commit. I use sqlite3 for my database (I plan to change later, but for now I want to keep it simple) and I stored some of my settings variable in a .env file.

Here is my .gitlab-ci.yml file (I used the default file proposed by gitlab and removed what I thought I didn't need) :

image: python:latest

variables:
    SECRET_KEY: "this-is-my-secret-key"
    DEBUG: "True"
    ALLOWED_HOSTS: "['*']"
    DB_ENGINE: "django.db.backends.sqlite3"
    DB_NAME: "test_database.sqlite3"
    STATIC_URL: "/static/"

cache:
  paths:
    - ~/.cache/pip/

before_script:
  - python -V 
  - pip install -r requirements.txt

test:
  script:
    - cd backend
    - python3 manage.py makemigrations
    - python3 manage.py migrate
    - python3 manage.py test

but when I commit, I get this error

$ python3 manage.py makemigrations
/usr/local/lib/python3.9/site-packages/environ/environ.py:628: UserWarning: /builds/romainros/ynoverflow/backend/ynoverflow/.env doesn't exist - if you're not configuring your environment separately, create one.
  warnings.warn(
Traceback (most recent call last):
  File "/builds/romainros/ynoverflow/backend/manage.py", line 22, in <module>
    main()
  File "/builds/romainros/ynoverflow/backend/manage.py", line 18, in main
    execute_from_command_line(sys.argv)
  File "/usr/local/lib/python3.9/site-packages/django/core/management/__init__.py", line 381, in execute_from_command_line
No changes detected
    utility.execute()
  File "/usr/local/lib/python3.9/site-packages/django/core/management/__init__.py", line 375, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/usr/local/lib/python3.9/site-packages/django/core/management/base.py", line 336, in run_from_argv
    connections.close_all()
  File "/usr/local/lib/python3.9/site-packages/django/db/utils.py", line 224, in close_all
    connection.close()
  File "/usr/local/lib/python3.9/site-packages/django/db/backends/sqlite3/base.py", line 248, in close
    if not self.is_in_memory_db():
  File "/usr/local/lib/python3.9/site-packages/django/db/backends/sqlite3/base.py", line 367, in is_in_memory_db
    return self.creation.is_in_memory_db(self.settings_dict['NAME'])
  File "/usr/local/lib/python3.9/site-packages/django/db/backends/sqlite3/creation.py", line 12, in is_in_memory_db
    return database_name == ':memory:' or 'mode=memory' in database_name
TypeError: argument of type 'PosixPath' is not iterable

I've tried to add a app.settings.ci file and add --settings app.settings.ci in my test command, but I got this error :

django.core.exceptions.ImproperlyConfigured: Set the SECRET_KEY environment variable

Any idea ?

EDIT

I use django-environ for my environment variables.

Here are the most important parts of my settings

# settings.py

from datetime import timedelta

import environ
from pathlib import Path

env = environ.Env()
# reading .env file
environ.Env.read_env()



# SECURITY WARNING: keep the secret key used in production secret!
SECRET_KEY = env("SECRET_KEY")

# SECURITY WARNING: don't run with debug turned on in production!
DEBUG = env("DEBUG")

ALLOWED_HOSTS = env("ALLOWED_HOSTS")


# Application definition

INSTALLED_APPS = [
    'django.contrib.admin',
    'django.contrib.auth',
    'django.contrib.contenttypes',
    'django.contrib.sessions',
    'django.contrib.messages',
    'django.contrib.staticfiles',
    'rest_framework',
    'rest_framework_simplejwt.token_blacklist',
    'api'
]

# Database
# https://docs.djangoproject.com/en/3.1/ref/settings/#databases

DATABASES = {
    'default': {
        'ENGINE': env("DB_ENGINE"),
        'NAME': BASE_DIR / env("DB_NAME"),
    }
}

STATIC_URL = env("STATIC_URL")