Update unix instructions for x-compiling on ARM (#5811)

docs/building/unix-instructions.md

@@ -46,6 +46,7 @@ Cross compilation is only supported on an Ubuntu host, 18.x and newer, and only
 ```sh
 sudo apt-get update
 sudo apt-get install debootstrap qemu-user-static
+sudo apt-get install binutils-aarch64-linux-gnu
 sudo ./eng/common/cross/build-rootfs.sh <target architecture> <ubuntu distro name> --rootfsdir <new rootfs location>
 export ROOTFS_DIR=<new rootfs location>
 
@@ -55,6 +56,8 @@ export ROOTFS_DIR=<new rootfs location>
 
 Note that the `<target architecture>` will usually be arm or arm64 and the `<ubuntu distro name>` is bionic for 18.04.
 
+Alternatively, use the following Docker image which contains all the software packages and configurations required to cross-compile for ARM `mcr.microsoft.com/dotnet-buildtools/prereqs:ubuntu-18.04-mlnet-cross-arm64-20210519131124-2e59a5f`.
+
 ### macOS
 
 macOS 10.13 (High Sierra) or higher is needed to build dotnet/machinelearning. We are using a .NET Core 3.1 SDK to build, which supports 10.13 or higher.
@@ -73,4 +76,4 @@ $ brew update && brew install cmake https://raw.githubusercontent.com/dotnet/mac
 
 Please note that newer versions of Homebrew [don't allow installing directly from a URL](https://github.com/Homebrew/brew/issues/8791). If you run into this issue, you may need to download libomp.rb first and install it with the local file instead.
 
-Also, libomp version 7.0.0 doesn't have a cask for Big Sur. You can work around this by downloading the libomp.rb file and then calling `brew install libomp.rb --build-from-source --formula`.
+Also, libomp version 7.0.0 doesn't have a cask for Big Sur. You can work around this by downloading the libomp.rb file and then calling `brew install libomp.rb --build-from-source --formula`.