chore: update DATABASE_URL to use the database created via POSTGRES_D… (#4836)

…B in compose

## About the changes
I've noticed that all migrations are applied to the `postgres` database
along we've the variable POSTGRES_DB=db in docker compose.
The reason is the DATABASE_URL variable still refers to `postgres`
instead `db`. I updated the URL and now it works with `db`.

docker-compose.yml

@@ -23,7 +23,7 @@ services:
       - "4242:4242"
     environment:
       # This points Unleash to its backing database (defined in the `db` section below)
-      DATABASE_URL: "postgres://postgres:unleash@db/postgres"
+      DATABASE_URL: "postgres://postgres:unleash@db/db"
       # Disable SSL for database connections. @chriswk: why do we do this?
       DATABASE_SSL: "false"
       # Changing log levels: