Small improvements to README file based on recent experience

docker-compose.yml

@@ -4,6 +4,7 @@ services:
 
   db:
     image: postgres:12.0
+    env_file: .env
     environment:
       - "POSTGRES_HOST_AUTH_METHOD=trust"
     networks:
@@ -11,7 +12,7 @@ services:
     volumes:
       - postgres_data:/var/lib/postgresql/data/
     ports:
-      - "5432:5432"
+      - "$PGPORT:5432"
 
   redis:
     image: "redis:alpine"