docker compose with auto volume

docker-compose.yml

version: '3.4'

services:
  marathontimetracker.server:
    image: ${DOCKER_REGISTRY-}marathontimetrackerserver
    build:
      context: .
      dockerfile: MarathonTimeTracker.Server/Dockerfile
    environment:
        - DB_PW
    depends_on:
        - db
  db:
    image: mcr.microsoft.com/mssql/server
    volumes:
      - type: volume
        source: db_data
        target: /var/opt/mssql
    environment:
      SA_PASSWORD: "${DB_PW}"
      ACCEPT_EULA: "Y"
    ports:
      - "1433:1433"
volumes:
    db_data: {}