Creating a docker container for development environment

Make it easy for devs familiar with docker to 'hit the ground running' with a dev docker image.

Add it to https://freecad.github.io/DevelopersHandbook/gettingstarted/

Start with the gettingstarted section linked in the issue and review how development setup instructions are organized. Define the development Docker image and document how to use it there; the work is done when Docker-familiar developers can start the FreeCAD development environment from the handbook instructions.

Written by the indexing model from the issue text.