Add a note on building in a docker image instead

[n-hutton]

The note should be pretty self-explanatory.

It should be simple for the reviewer to test this works. Delete a bit file and follow this method to recreate it. I don't know if there is a proxmark docker repo, but if there is, it should be a simple matter to retag it and push it to there, instead.

Tested on M1 mac and x86 ubuntu.

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Perhaps there was an easier way, but...

For future reference to anyone reading this PR, to install Xilinx WebPack ISE 10.1 it was downloaded as a zip, mounted into a linux container and then during the install the gui installation popups were forwarded back to the host (ubuntu 22.04.1) with the following command:

docker run -v <LOCAL_PATH>:/tmp --rm -it -v $XAUTHORITY:/tmp/.XAuthority -e XAUTHORITY=/tmp/.XAuthority --net=host -e DISPLAY=:0 i686/ubuntu bash

Inside the container, the linux32 command was needed to trick the install process into being happy about the running architecture. This was only required for the install.

The docker image after the install can be 'frozen' using docker commit which allows you to restart it as it was (minus environment variables which have to be added to the .bashrc).

[github-actions[bot]]

You are welcome to add an entry to the CHANGELOG.md as well