To configure the database, postgres's default environment variables must be set.
Add them in a file called ".env".
DB_NAME=
DB_USER=
DB_PASSWORD=
Also add these. DOMAIN_URL consists of scheme + domain + port (if not the default ones; 80/443 )
DOMAIN=
DOMAIN_URL=
To enable 42.fr API also add these to ".env".
API_42_UID=
API_42_SECRET=
For 42's API to work it is necessary to add the DOMAIN_URL + route to oauth callback (oauth/callback/) in the field "Redirect URI" in your 42 application (V2).
In order to fully erase database's contents (if postgres's user is not mapped on your system) make purgedb can be run.