How do I add PostGIS to my PostgreSQL setup using pure docker-compose

Why not use Dockerfile specfic to add postgis?

Change your "image" property to "build" property to link a Dockerfile.

Your docker-compose.yml would look like this:

version: '3'
services:
  db:
    container_name: db
    build:
      context: .
      dockerfile: Dockerfile-db
    restart: always
    environment:
      POSTGRES_USER: user
      POSTGRES_PASSWORD: pass
      POSTGRES_DB: db_name
    ports:
      - "5432:5432"

Then create a text file named Dockerfile-db in the same path as docker-compose.yml with the following content:

FROM postgres:14.1


RUN apt-get update && apt-get  install -y postgresql-14-postgis-3  


CMD ["/usr/local/bin/docker-entrypoint.sh","postgres"]

notice that you can specify the PostGIS version that goes along with your Postgresql version.

This way you can install all you need in the container by extending the Dockerfile-db

Notice: your db container now has PostGIS but we need to create the extension in the DB and for that you should run CREATE EXTENSION postgis; you only need to do this once if you use volumes.

Bonus: One way of doing it's by preparing a directory called db/ with init.sql file and write something like CREATE EXTENSION postgis ; and persist it as a volume in docker compose so the final format for your docker-compose would be :

version: '3'
services:
  db:
    container_name: db
    build:
      context: .
      dockerfile: Dockerfile-db
    restart: always
    environment:
      POSTGRES_USER: user
      POSTGRES_PASSWORD: pass
      POSTGRES_DB: db_name
    ports:
      - "5432:5432"
    volumes:
      - ./db:/docker-entrypoint-initdb.d/
      - database_volume:/var/lib/postgresql/data

volumes:
  database_volume:

this way the postgresql will run this script and create your extensions