dockerTools: normalize arch to GOARCH

Docker (via containerd) and the the OCI Image Configuration imply and
suggest, respectfully, that the architecture set in images matches those
of GOARCH in the Go Language document.

This changeset updates the implimentation of getArch in dockerTools to
return GOARCH values, to satisfy Docker.

Fixes: #106695

nixos/tests/docker-tools.nix

@@ -245,7 +245,7 @@ import ./make-test-python.nix ({ pkgs, ... }: {
                 "docker inspect ${pkgs.dockerTools.examples.cross.imageName} "
                 + "| ${pkgs.jq}/bin/jq -r .[].Architecture"
             ).strip()
-            == "${if pkgs.system == "aarch64-linux" then "amd64" else "arm64v8"}"
+            == "${if pkgs.system == "aarch64-linux" then "amd64" else "arm64"}"
         )
   '';
 })