fix(examples): do not publish postgres port

Only the broker needs access to the postgres instance, so there is no need
to publish the ports. One big advantage of this is, that if the user
already runs a postgres instance the service inside docker will not interfere
with it and the broker will start correctly.

examples/docker-compose.yml

@@ -3,8 +3,6 @@ version: "3.9"
 services:
   postgres:
     image: postgres
-    ports:
-      - "5432:5432"
     healthcheck:
       test: psql postgres -U postgres --command 'SELECT 1'
     environment: