[#49] Add db integration tests

.gitlab-ci.yml

Makefile

README.md

docker-compose.test.yml

tests/integration/__init__.py

tests/integration/conftest.py

tests/integration/test_postgres_data_source.py

tests/unit/__init__.py

tests/unit/test_postgres_data_source.py

tests/unit/test_router.py

tests/unit/test_utils.py