unix: support building Linux x86-64 from macOS aarch64#1179
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Docker on aarch64 macOS will automagically virtualize x86-64 containers if containers are spawned with `platform=linux/amd64`. Performance of spawned containers is a bit slower than native, but not horrible. This functionality means it is viable to develop Linux x86-64 from modern Apple hardware. This commit teaches the build system to support cross-compiling Linux x86-64 from macOS aarch64. Implementing this wasn't too difficult: we need to pass `platform` into Docker's APIs for building and creating containers. We need to teach code to resolve the effective host platform when this scenario is detected. And we need to advertise support for cross-compiling in the `targets.yml` file. In case you are wondering, yes, a similar solution could be employed for Linux too by using emulation. But this requires Docker be configured to support emulation, which isn't common. Rosetta on macOS "just works" and is therefore the lowest hanging fruit to implement.
Subprocess out to docker buildx to build images. The api.build function does not support BuildKit and always uses the legacy builder. To properly support the platform argument BuildKit must be used.
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Update to #1056 to use
docker buildxfor reliable handeling of theplatformargument.