version: "3.2"
services:
  db:
      container_name: postgresserver
      image: mdillon/postgis
      ports:
          - "54322:5432"
      environment:
          POSTGRES_DB: "postgres"
          POSTGRES_USER: "postgres"
          POSTGRES_PASSWORD: "postgres"

  db2:
      extends: db
      container_name: postgresserver2
      environment:
          POSTGRES_DB: "postgres2"
          POSTGRES_USER: "postgres2"
          POSTGRES_PASSWORD: "postgres2"

ERROR: The Compose file '.docker-compose.yml' is invalid because:
Unsupported config option for services.db2: 'extends'