Docker Compose - How to store database data?

docker-compose down

does the following

Stops containers and removes containers, networks, volumes, and images created by up

So the behaviour you are experiencing is expected.

Use docker-compose stop to shutdown containers created with the docker-compose file but not remove their volumes.

Secondly you don't need the data-container pattern in version 2 of docker compose. So remove that and just use

  db:
    ...
    volumes:
       - /var/lib/postgresql/data

To store database data make sure your docker-compose.yml will look like if you want to use Dockerfile

version: '3.1'

services:
  php:
    build:
      context: .
      dockerfile: Dockerfile
    ports:
      - 80:80
    volumes:
      - ./src:/var/www/html/
  db:
    image: mysql
    command: --default-authentication-plugin=mysql_native_password
    restart: always
    environment:
      MYSQL_ROOT_PASSWORD: example
    volumes:
      - mysql-data:/var/lib/mysql

  adminer:
    image: adminer
    restart: always
    ports:
      - 8080:8080
volumes:
  mysql-data:

your docker-compose.yml will looks like if you want to use your image instead of Dockerfile

version: '3.1'   

services:
  php:
    image: php:7.4-apache
    ports:
      - 80:80
    volumes:
      - ./src:/var/www/html/
  db:
    image: mysql
    command: --default-authentication-plugin=mysql_native_password
    restart: always
    environment:
      MYSQL_ROOT_PASSWORD: example
    volumes:
      - mysql-data:/var/lib/mysql

  adminer:
    image: adminer
    restart: always
    ports:
      - 8080:8080
volumes:

if you want to store or preserve data of mysql then must remember to add two lines in your docker-compose.yml

volumes:
  - mysql-data:/var/lib/mysql

and

volumes:
  mysql-data:

after that use this command

docker-compose up -d

now your data will persistent and will not be deleted even after using this command

docker-compose down

extra:- but if you want to delete all data then you will use

docker-compose down -v

to verify or check database data list by using this command

docker volume ls

DRIVER              VOLUME NAME
local               35c819179d883cf8a4355ae2ce391844fcaa534cb71dc9a3fd5c6a4ed862b0d4
local               133db2cc48919575fc35457d104cb126b1e7eb3792b8e69249c1cfd20826aac4
local               483d7b8fe09d9e96b483295c6e7e4a9d58443b2321e0862818159ba8cf0e1d39
local               725aa19ad0e864688788576c5f46e1f62dfc8cdf154f243d68fa186da04bc5ec
local               de265ce8fc271fc0ae49850650f9d3bf0492b6f58162698c26fce35694e6231c
local               phphelloworld_mysql-data

docker-compose down stops containers but also removes them (with everything: networks, ...).

Use docker-compose stop instead.

I think the best approach to make containers that can store database's data with docker-compose is to use named volumes:

version: '2'
services:

  db: #https://hub.docker.com/_/mysql/
    image: mysql
    volumes:
      - "wp-db:/var/lib/mysql:rw"
    env_file:
      - "./conf/db/mysql.env"

volumes:
  wp-db: {}

Here, it will create a named volume called "wp-db" (if it doesn't exist) and mount it in /var/lib/mysql (in read-write mode, the default). This is where the database stores its data (for the mysql image).

If the named volume already exists, it will be used without creating it.

When starting, the mysql image look if there are databases in /var/lib/mysql (your volume) in order to use them.

You can have more information with the docker-compose file reference here: https://docs.docker.com/compose/compose-file/#/volumes-volume-driver