Why I don't lose postgresql data when rebuild docker image?

Question

version: '3'

services:
  db:
    image: postgres
  web:
    build: .
    command: python3 manage.py runserver 0.0.0.0:8000
    volumes:
      - .:/code
    ports:
      - "8000:8000"
    depends_on:
      - db

Why I don't lose data when running docker-compose build --force-em --no-cache. If this is normal, why do we need to create volume for data folder ?

Answer 1

When running the command docker-compose build --force-em --no-cache, this will only build the web Docker image from the Dockerfile which in your case is in the same directory.

This command will not stop the containers that you have previously started using this compose file, thus you want lose any data when running this command.

However, as soon as you remove the containers using docker-compose down or when containers are stopped docker-compose rm, you won't find the postgres data when you restart the container.

If you want to persist the data, and make the container pick it up when it is recreated, you need to give the postgres data volume a name as such.

version: '3'

services:
  db:
    image: postgres
    volumes:
      - pgdata:/var/lib/postgresql/data
  web:
    build: .
    command: python3 manage.py runserver 0.0.0.0:8000
    volumes:
      - .:/code
    ports:
      - "8000:8000"
    depends_on:
      - db

Now the postgres data won't be lost when the containers are recreated.