Where are the default Docker Hub automated build steps?

Question

I've overriden the test step of automated builds to support DOCKER_BUILDKIT after a bit of trial & error by overriding this step via a custom hook hooks/test ...

#! /bin/bash

DOCKER_BUILDKIT=1 \
    docker build \
    -f $DOCKERFILE_PATH \
    -t $IMAGE_NAME \
    .

... but I don't know what the default dockerhub test step is.

I want to run sut command to run my test suite after building the sut image like the default test behaviour -

  ...
  sut:
    build: .
    volumes:
      - .:/app
    command: pytest
    depends_on:
      - db
      - mssql
      - selenium

Ref Where are the default Docker Hub automated build steps?

Ref Automated builds with moby/buildkit not working overall · Issue #1918 · docker/hub-feedback (github.com)

Ref Automated repository tests | Docker Documentation