Problem with Docker Compose - FATAL: database "root" does not exist (PostgresSQL)

Question

I trying to create a Postgres Database with Docker Compose, but i'm not understanding why the PSQL client is telling me that a database called root doesn't exists.

That's the docker-compose.yml

version: '3.3'

services:
  postgres:
    container_name: postgres
    image: "library/postgres:latest"
    env_file:
      - .env # configure postgres
    volumes:
      - database-data:/var/lib/postgresql/data/ # persist data even if container shuts down
    environment:
      POSTGRES_PASSWORD: ${DB_PASSWORD}
      POSTGRES_USER: ${DB_USER}
      POSTGRES_DB: ${DB_DATABASE}
    ports:
      - "5432:5432"
    restart: unless-stopped

volumes:
  database-data:

When i run docker-compose exec postgres env it gives the right variables

POSTGRES_PASSWORD=4544
POSTGRES_USER=root
POSTGRES_DB=xpack

And when i run docker exec -it postgres /bin/bash and psql it returns the following error:

root@0719bff80472:/# psql -U root
psql: error: could not connect to server: FATAL:  database "root" does not exist

I don't if my database was created or not