I am setting up a database for running integration tests. In my docker-compose.yml file I have:

services:
  ...
  postgres:
    image: "postgres:16"
    volumes:
      - "postgres:/var/lib/psql/data"
    env_file: ".env"
    ports:
      - "5432:5432"

volumes:
  postgres:

In my docker-compose.dev.yml I have only this:

services:
  postgres:
    volumes:
      - "test_postgres:/var/lib/psql/data"

volumes:
  test_postgres:

The premise is it’s the same container that mounts a different volume if run with docker compose -f docker-compose.yml -f docker-compose.dev.yml up postgres. However the changes I make on one volume persist on the other. Is this even possible?

Here’s the docker config output:

$ docker compose -f docker-compose.yml -f docker-compose.dev.yml config postgres
name: mercury
services:
  postgres:
    environment:
      POSTGRES_DB: mercury
      POSTGRES_PASSWORD: mercury
      POSTGRES_USER: mercury
    image: postgres:16
    networks:
      default: null
    ports:
      - mode: ingress
        target: 5432
        published: "5432"
        protocol: tcp
    volumes:
      - type: volume
        source: test_postgres
        target: /var/lib/psql/data
        volume: {}
networks:
  default:
    name: mercury_default
volumes:
  test_postgres:
    name: mercury_test_postgres

$ docker compose -f docker-compose.yml config postgres
name: mercury
services:
  postgres:
    environment:
      POSTGRES_DB: mercury
      POSTGRES_PASSWORD: mercury
      POSTGRES_USER: mercury
    image: postgres:16
    networks:
      default: null
    ports:
      - mode: ingress
        target: 5432
        published: "5432"
        protocol: tcp
    volumes:
      - type: volume
        source: postgres
        target: /var/lib/psql/data
        volume: {}
networks:
  default:
    name: mercury_default
volumes:
  postgres:
    name: mercury_postgres

Note the volumes’ source and target.