Apache Spark Docker image repository

[Dongjoon Hyun]

Hi, All.

>From 2020, shall we have an official Docker image repository as an
additional distribution channel?

I'm considering the following images.

    - Public binary release (no snapshot image)
    - Public non-Spark base image (OS + R + Python)
      (This can be used in GitHub Action Jobs and Jenkins K8s Integration
Tests to speed up jobs and to have more stabler environments)

Bests,
Dongjoon.