steps:
  - name: ":rspec:"
    command: "rspec --color specs"
    env:
      BUILDKITE_DOCKER_COMPOSE_CONTAINER: "app"

steps:
  - name: ":rspec:"
    command: "rspec --color specs"
    plugins:
      docker-compose#v2.5.0:
        run: app

Update to Docker Compose plugin
Update to Docker Compose plugin