Build arm64 image variations for the debian and ubuntu definitions for M1 macs

[Chuxel]

As described in #558 (comment), the situation for Docker on M1 macs is not ideal since both debian:buster nor ubuntu:focal experience segmentation faults due to an issue in libss1.1. It seems unlikely that buster will be patched now that bullseye is out (and buster therefore gets security fixes only). Unclear about Ubuntu 20.04/Focal.

Therefore this PR:

1. Adds arm64 builds for Debian bullseye and stretch, but leaves buster as x86 only.
2. Adds arm64 builds for Ubuntu bionic, but leaves focal as x86 only.
3. Breaks our typical policy of focusing on LTS releases considering the situation and adds an Ubuntu 21.04 / hirsute variant with arm64 support.

//cc: @joshspicer @2percentsilk @bamurtaugh @chrmarti

We can then move on to other images from here given many already have bullseye image variants we can use.