Replace docker-py with python-on-whales

[0xricksanchez]

Use python-on-whales instead of docker-py as the former supports building with buildkit/buildx

[0xricksanchez]

docker-py looked abandoned when I opened this issue, but it seems that there's some activity again with a first new release on the horizon. Waiting this out just a little longer before committing to the switch to python-on-whales