Add support for linux/arm64

[Frank Schröder (magiconair)]

This patch updates the Makefile to build a linux/amd64 and linux/arm64 image and tag the linux/amd64 image with the current name.

The .circleci build script has been updated to create a docker multi-architecture manifest. The images need to get pushed before the manifest can be created.

Fixes #40

[Frank Schröder (magiconair)]

I'll test this now in our environment.

[Frank Schröder (magiconair)]

First check is that the container works well for both linux/amd64 and linux/arm64

[Frank Schröder (magiconair)]

But I'll need some help with the build environment.