fix: postgres columns with timezone

Add timezone for the columns creation_date and update_date, otherwise
it's difficult to compare the times correctly in the unit test.

Furthermore the default value of the creation_date will now be defined
by the postgres implementation of the database interface.

docker-compose.yml

@@ -6,7 +6,7 @@ services:
     - PGTZ=Europe/Berlin
     - POSTGRES_PASSWORD=postgres
     - TZ=Europe/Berlin
-    image: postgres:11.5-alpine
+    image: postgres:13-alpine
     ports:
     - 5432:5432
     restart: always