Aarch64 base image is not available anymore

[lhoupert]

I recently try to re-build the docker image for my raspberrypi and I encoutered the error below:

 > [internal] load metadata for docker.io/aarch64/alpine:3.5:
------
Dockerfile.aarch64:4
--------------------
   2 |     
   3 |     # Smallest base image
   4 | >>> FROM aarch64/alpine:3.5
   5 |     
   6 |     LABEL maintainer="Kyle Manna <kyle@kylemanna.com>"
--------------------
ERROR: failed to solve: aarch64/alpine:3.5: docker.io/aarch64/alpine:3.5: not found

I reaslised the aarch64 group doesnt exist anymore on dockerhub. arm64v8 should be used instead.

I raised a PR for fixing this here: #759