base-image: bump docker to 20.10.23-git, update build docs

security fixes

This fixes some critical CVEs in curl and git as docker hub reports by bumping the docker image to 20.10.23.

docs update

In !221 (merged) building the base image for armv6 was effectively broken by switching to docker 20.x which no more provides armv6 images (related: docker:20.10.16-git, docker:19.03.8-git). This leads to an error when adding buildx:

 > [linux/arm/v6 internal] load metadata for docker.io/library/docker:20.10.16-git:
------
Dockerfile:1
--------------------
   1 | >>> FROM docker:20.10.16-git
   2 |
   3 |     COPY --from=docker/buildx-bin /buildx /usr/libexec/docker/cli-plugins/docker-buildx
--------------------
ERROR: failed to solve: docker:20.10.16-git: no match for platform in manifest sha256:253e7e8c8259fbaa61ca4146331eaa7ee0fb05308c95b67aa38f7b5c68d3bb50: not found

Adjusting the docs accordingly.