aarch64 support

[jgowdy]

Is this script capable of building a rootfs for aarch64 using the files in http://dl-2.alpinelinux.org/alpine/v3.12/main/aarch64/? Does it have to be run from an aarch64 host to do so? It seems like there's a reference to an x64 specific package in

https://github.com/alpinelinux/apk-tools/releases/download/v2.10.4/apk-tools-2.10.4-x86_64-linux.tar.gz

I know this exists, but I would love to be able to build my own using your script:

alpine-minirootfs-3.12.0-aarch64.tar.gz

Any hints as to how to use or modify the script to support aarch64 would be greatly appreciated, thank you!