Build aarch64 Docker images in CI

[fwcd]

Ideally we could just build an aarch64 image too (under QEMU user-mode emulation) by using

platforms: linux/amd64,linux/arm64/v8

in .github/workflows/docker.yml.

While this works for most build steps, this unfortunately fails with obscure 'invalid manifest' errors when building D2 with the Swift Package Manager under QEMU. Further investigation revealed that these crashes occur due to SIGTRAPs sent to swift-driver while fetching/checking out D2's dependencies. This can be reproduced by building the image up to the step before swift build -c release and then manually running the Swift build e.g. in a shell within such a container. The errors will occur and QEMU will place a core dump (*.core) in the working directory (which can be inspected using lldb /usr/bin/swift-driver --core <path/to/dump.core> and the command bt all).

This bug also seems to be tracked here:

• https://forums.swift.org/t/build-crash-when-building-in-qemu-using-new-swift-5-6-arm64-image/56090/4
• Swift support on arm64 rvolosatovs/docker-protobuf#89

Possible ways to resolve this issue:

• Use a GitHub-provisioned native ARM runner once available
• Try using full system emulation for aarch64 with QEMU with Docker
  • It might not be trivial to set this up, most documentation seems to be focused
    on running QEMU under user-mode emulation when building multiarch images
  • Docker Desktop seemed to do something in this direction on M1 Macs prior to using the Virtualization framework
  • A drackback would be that this is probably quite a bit slower
  • https://reverseengineering.stackexchange.com/questions/15015/qemu-gdb-server-thread-problem
    seems to deal with a similar problem.
  • The section Using machine emulation in https://wiki.ubuntu.com/ARM/RootfsFromScratch/QemuDebootstrap might be helpful
• Investigate whether we can cross-compile Swift from x86_64 directly within the Docker build
  • Create multiarch image by cross-compiling Swift package to arm64 #125 tracks the current implementation of this
  • This would probably be the best (and fastest) approach, but might be non-trivial,
    depending on how well the Swift Package Manager supports cross-compiling from x86_64 Linux hosts to
    aarch64 Linux.
  • https://www.docker.com/blog/faster-multi-platform-builds-dockerfile-cross-compilation-guide/
    is a guide that explains how to do this (in general)
  • feat: add support for cross-compilation rvolosatovs/docker-protobuf#91 does something similar