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
