Multiple Postgres databases in a single Docker container

This will be a short one.

The official Postgres Docker image supports a few environment variables. One of them, POSTGRES_DB, is responsible for holding a database name. However, if you want your container to include more than one database (e.g app and app_test), you have to reach for different solutions.

One of them is to create a bash script that sets up multiple databases by psql command. Postgres will execute it on database startup inside the container. An example of such script, create-multiple-postgresql-databases.sh, can be found here. Credits to mrts at Github.

You can place it in whatever project directory you want, although pg-init-scripts/ might be a good starting point. Then, to make the script visible and executable by Postgres, it has to be mounted to the /docker-entrypoint-initdb.d container directory.

To mount a host directory to a container directory, we have to use volumes.

Syntax: <host directory>:<container directory>.

WARNING: You cannot mount a single file to a directory, it works just for directories.

Final docker-compose.yml:

version: "3.7"
services:
  db:
    container_name: "app_db"
    image: "postgres:11.5-alpine"
    volumes:
      - ./pg-init-scripts:/docker-entrypoint-initdb.d
    environment:
      - POSTGRES_USER=${DB_USER}
      - POSTGRES_PASSWORD=${DB_PASSWORD}
      - POSTGRES_MULTIPLE_DATABASES=app,app_test

The ${DB_USER} syntax means that the DB_USER value is read from automatically loaded .env file placed inside the same directory as docker-compose.yml.

Corresponding .env:

DB_USER=someuser
DB_PASSWORD=somepassword

Remember, you shouldn't commit .env files! Create and commit .env-example file with empty secret values instead.

If you want to use a different path (e.g .env-docker), define env_file in docker-compose.yml:

env_file:
  - ./.env-docker

Now, with every piece in place, run docker-compose up to start the service.

Happy coding!