Setting up a docker container for Postgres, how fun! There are a few issues that I have run into repeatedly when doing this. The first is that I usually have Postgres running on the local machine which has already taken over the default port of 5432. To map this port differently, use the ports command. You also need to set up (1) a persistent volume outside the container and (2) the primary user credentials. At the end of the day, this is what my docker-compose service looks like for Postgres:

    db:
      image: postgres
      ports: 
        - "5433:5432"
      environment:
        - POSTGRES_DB=postgres
        - POSTGRES_USER=postgres
        - POSTGRES_PASSWORD=postgres
      networks:
        - bridgenetwork
      volumes:
        - ./postgres-data:/var/lib/postgresql/data

This assumes that you want the default db/user/password to be postgres and you have a directory, outside the container and in the current directory called postgres-data which will be mapped into the container to the place where Postgres stores it data (/var/lib/postgresql/data).

Oh, and your default network is called bridgenetwork

Now, what if you want a different docker container (not part of this docker compose file) to use this container? For example, I usually run my local PostgresSQL through docker and now have a different project that wants to access it?

The key is to change the host on the machine that wants to access the PostgresSQL container to host = 'host.docker.internal'. The example below would be able to connect to the database above since it uses the default port of 5432.

    host = 'host.docker.internal'
    user_name = 'postgres'
    password = 'postgres'
    dbname = 'postgres'

    conn_string = "host='%s' dbname='%s' user='%s' password='%s'" % (host, dbname, user_name, password)
    postgres_conn = psycopg2.connect(conn_string)