Document the process to build Go application targeting an old glibc and cross-build using .NET shared images

• Document info originally from https://github.com/microsoft/go-lab/issues/60

To make it easier to read (and public 🙂), here are the most critical parts from my raw notes about how I got a program to depend on an old glibc and how to build systemcrypto cross-arch.

Overview of this approach: .NET Linux build methodology

https://github.com/dotnet/dotnet-buildtools-prereqs-docker builds the images that include a rootfs.

Usage guides:
https://github.com/dotnet/runtime/blob/main/docs/workflow/using-docker.md
https://github.com/dotnet/runtime/blob/main/src/coreclr/nativeaot/docs/containers.md

I used meta-build code like this to drive a properly configured go build command:

cmd.Env = append(
	os.Environ(),
	"GOEXPERIMENT=systemcrypto",
	"CGO_ENABLED=1",
	"GOOS="+goOS,
	"GOARCH="+goArch,
)
if *sysroot != "" {
	var linuxArch string
	switch goArch {
	case "amd64":
		linuxArch = "x86_64"
	case "arm64":
		linuxArch = "aarch64"
	default:
		return fmt.Errorf("unsupported architecture %q for sysroot", goArch)
	}
	ccPlatform := linuxArch + "-linux-gnu"
	cmd.Env = append(
		cmd.Env,
		// Set up for the compiler we expect to see in azurelinux-3.0-cross-* images.
		"CC="+joinSpace(
			"clang",
			"--target="+ccPlatform,
			"--sysroot="+*sysroot,
		),
		"CGO_LDFLAGS="+joinSpace(
			// Use lld: ld is not present.
			"-fuse-ld=lld",
		),
	)
}

In the pipeline yml, I used these containers:

    containers:
      cross-amd64:
        image: mcr.microsoft.com/dotnet-buildtools/prereqs:azurelinux-3.0-net10.0-cross-amd64
      cross-arm64:
        image: mcr.microsoft.com/dotnet-buildtools/prereqs:azurelinux-3.0-net10.0-cross-arm64

On linux amd64: -sysroot /crossrootfs/x64
On linux arm64: -sysroot /crossrootfs/arm64

Start with the linked .NET Linux build methodology, Docker prereqs repository, and runtime container guides. Use the provided Go environment snippet, pipeline YAML, cross-image names, and sysroot paths as source material for a public guide covering old-glibc and amd64/arm64 systemcrypto builds. Done means the process, commands, links, and platform-specific paths are documented.