Switch to using docker buildx

[yuvipanda]

We're using docker-py with the old build infrastructure, which hasn't really been updated in years. We should move to using buildx, which unfortunately is only available via shelling out (docker/docker-py#2230). But it allows us a lot of positive functionality (like https://docs.docker.com/build/builders/drivers/kubernetes/) that probably will help with some of the building issues we now see with the 2i2c hetzner install

[manics]

@ryanlovett has tried buildx with repo2podman #875 (comment)

[minrk]

Nice! I think thanks to your work @manics on creating the Engine API, this might not be too much. We can drop docker-py altogether, which seems sadly abandoned.

[ryanlovett]

repo2podman has been working nicely. (thanks @manics!) BuildKit parallelizes multi-stage builds which speeds up images with large python and R stacks.

[yuvipanda]

Dupe of #875