* fixing nullable fields issue in Go avroio

* readability

* updating all references to goavro

* added support for mac-os, tested on mac-os 10.15.2, ubuntu 20.04

* output for user about "docker post-install steps" was added (mac case)

* hide error message, if docker group not exist

* fixed issue with text color

* [BEAM-10891] Standardized build environment using Docker

* Document additional python tools needed to build

* Add pyenv and pyenv-virtualenv to the docker image

* Attempt workaround wrong python distlib version

* Install extra tools

* Drop pyenv from the image