[Feature Request] Support docker build in qmk_userspace repos

[sotte]

Description

The main qmk firmware repository provides the handy "build via docker" features.
Here are the docs: https://docs.qmk.fm/getting_started_docker
Here is the implementation: https://github.com/qmk/qmk_firmware/blob/master/util/docker_build.sh

It would be great if qmk_userspace also supported ''build via docker".

(The github actions basically do something along the lines, but you cannot easily reproduce it locally.)

Potential implementation:

• download the qmk_firmware repo within the docker container
• point to a local qmk_firmware checkout

[sotte]

Thanks a lot @tzarc. Works like a charm.

I'm wondering if we should mention the userspace build in the "getting started with docker" documentation here.