How I run pg_dump in my docker-compose setup

I have the following docker-compose.yml service:

services:
  postgres:
    image: postgres
    restart: unless-stopped
    volumes:
      - ./pg_data:/var/lib/postgresql/data
    environment:
      - POSTGRES_PASSWORD=${POSTGRES_PASSWORD}
      - POSTGRES_DB=${POSTGRES_DB}
      - POSTGRES_USER=${POSTGRES_USER}

With the following .env:

POSTGRES_DB=headscale
POSTGRES_USER=headscale
POSTGRES_PASSWORD=vah2phuen3shesahc6Jeenaechecee

Given that setup, I run pg_dump like this:

source .env && docker-compose exec postgres pg_dump -U${POSTGRES_USER} > pgdump-$(date +%F_%H-%M-%S).sql