DOCS: missing installation instructions for the docker buildx plug-in

[thomas-fossati]

Reported by @gkostal:

One other hiccup I had with getting the docker version
running was that I needed to install the docker buildx
plug-in, and this wasn't mentioned in the configuration
instructions.

Configuration instructions stated:
sudo apt install git [docker.io](http://docker.io/) jq tmux
sudo usermod -a -G docker $USER
newgrp docker

To which I had to add:
      sudo apt install docker-buildx