docker-compose.yml for Postgres works either way, but not the expected one - postgresql

Question

I have the following docker-compose.yml for running Postgres with Docker:
version: '3.8'
services:
postgres:
image: postgres:14.2-alpine
environment:
POSTGRES_USER: ${POSTGRES_USER:-postgres}
POSTGRES_PASSWORD: ${POSTGRES_PASSWORD:-postgres}
POSTGRES_DB: mydatabasename
PGDATA: /data/mydatabasename
volumes:
- postgres:/data/postgres
ports:
- '5432:5432'
networks:
- postgres
restart: unless-stopped
pgadmin:
... placeholder
networks:
postgres:
driver: bridge
volumes:
postgres:
pgadmin:
It works. What I don't understand, is that these two combinations work:
PGDATA: /data/mydatabasename
volumes:
- postgres:/data/postgres
and
PGDATA: /data/postgres
volumes:
- postgres:/data/mydatabasename
But this does not work:
PGDATA: /data/mydatabasename
volumes:
- postgres:/data/mydatabasename
I would just get: error: database "mydatabasename" does not exist.
The latter was my first attempt connecting everything though. So I am wondering, why do both fields not map to the actual database name? Thanks for your help!

Answer 1

Docker Compose File
version: '3'
services:
postgres:
image: postgres:alpine
container_name: postgres
restart: always
posts:
- 5432:5432
volumes:
- ./data_backup:/var/lib/postgresql/data #mount the data locally
env_file:
- .env
networks:
- postgres-net
pgadmin:
image: dpage/pgadmin4:6
restart: always
ports:
- 8080:80
volumes:
- pgadmin:/var/lib/pgadmin
env_file:
- .env
depends_on:
- postgres
netwroks:
- postgres-net
networks:
postgres-net:
name: postgres-net
.env File
#postgres
POSTGRES_PASSWORD="secure_password"
POSTGRES_DB=your_db_name
POSTGRES_USER=db_user_name
#pgadmin
PGADMIN_DEFAULT_EMAIL=ariful#firora.com
PGADMIN_DEFAULT_PASSWORD="secure_password"
PGADMIN_LISTEN_PORT=80
Since you may try to bind the database folder which was never created. Alternatively you can bind /var/lib/postgresql/data this path to store whatever database you created or about create in future.