Prisma/ User `postgres` was denied access on the database `practice.public` - prisma

Question

I'm quite new to Prisma, so if I overlook something basic, please forgive me.
I was running postgreSQL DB on a docker container and tried npx prisma migrate dev from local.
However, the following error occurred:
Error: P1010: User `postgres` was denied access on the database `practice.public`
my docker-compose.yml file and .env file were the followings:
// docker-compose.yml
version: "3"
services:
postgres:
image: postgres:14.1
container_name: postgres
hostname: postgres
ports:
- 5432:5432
environment:
POSTGRES_USER: postgres
POSTGRES_PASSWORD: postgres
POSTGRES_DB: practice
volumes:
- ./db:/var/lib/postgresql/data
restart: unless-stopped
// .env
DATABASE_URL="postgresql://postgres:postgres#localhost:5432/practice?schema=public"
I tried everything I could come up with, and accidentally found a solution.
First, I checked the username of my host computer.
echo $USER
>myusername
Then put the username into the DATABASE_URL in .env file, instead of the POSTGRES_USER I defined in docker-compose.yml.
// .env
DATABASE_URL="postgresql://myusername:postgres#localhost:5432/practice?schema=public"
npx prisma dev migrate worked for some reason. Congrats.
But I haven't the least idea of why it worked……
If you shed some lights on it, I would really appreciate it.