This can happen when you use a CircleCi pre-built service image as your primary container, such as Postgres or MongoDB. Rather, you should use a language image as the primary container, and follow that with your service images.
- image: circleci/postgres:9.6.2 - image: circleci/python:3.6.3
- image: circleci/python:3.6.3
- image: circleci/postgres:9.6.2