implement test backend and docker-based test

.gitlab-ci.yml

docker-compose.test.yml

docker-compose.yml

tests/test-backend/.cruft.json

tests/test-backend/.cruft.json.license

tests/test-backend/.gitattributes

tests/test-backend/.gitignore

tests/test-backend/.readthedocs.yaml

tests/test-backend/.reuse/shortcuts.yaml

tests/test-backend/.vscode/settings.json

tests/test-backend/.vscode/settings.json.license

tests/test-backend/CHANGELOG.md

tests/test-backend/Dockerfile

tests/test-backend/MANIFEST.in

tests/test-backend/Makefile

tests/test-backend/README.md

tests/test-backend/app.sh

tests/test-backend/conftest.py

tests/test-backend/django/Dockerfile

tests/test-backend/django/README.md