Creating a docker container for development environment

[luzpaz]

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/

[adrianinsaval]

there are dockerfiles in the main repo but I think they need some love to get them up to date, with the recent conda+VS stuff that oursland submitted it might be a better idea to push conda as a reproducible build env instead since that might work for windows and macOS too.

[luzpaz]

ref: https://github.com/FreeCAD/FreeCAD/tree/master/tools/build/Docker