postgres readyness check could be done with depends_on

right now we use a homegrown check if postgress is ready in the entrypoint of our django container https://gitlab.gwdg.de/era-public/plan/-/blob/develop/compose/django/entrypoint (originating from cookiecutter django). this could also be done in a more standard way, defining a healthcheck in the postgres container and depends_on in the compose file (read https://www.baeldung.com/linux/docker-compose-container-interdependence). implementing healthchecks is always a good thing do do :-)