buildkit-qemu-* binaries

[rittneje]

As briefly mentioned in https://github.com/moby/buildkit/blob/master/docs/multi-platform.md, the buildkit image contains a bunch of buildkit-qemu-* binaries under /usr/bin, which somehow get automatically invoked when building an image for a foreign architecture. However, we have some questions about this, and it would be helpful to update the docs to include this information.

1. How specifically does buildkit decide to automatically invoke these binaries? They are not going through binfmt_misc.
2. Will these binaries be used if qemu was also registered with the host kernel via binfmt_misc?
3. Where did these binaries come from? The ones in the latest image reports a version of 7.1.0, which is fairly old. And they also don't appear to be the same as the real qemu-*-static binaries available from https://github.com/multiarch/qemu-user-static/releases.
4. Is there a way to use a newer version of qemu than is available in the latest buildkit image?

[thompson-shaun]

Converting to a discussion based on the content. We can also convert back if this evolves.