[FR] Use ubuntu 20.04 as base image for Docker

[samos123]

EOL of Ubuntu 16.04 will April 2021

[bendory]

Thanks for the request. We don't have plans to change the base image at this time but I'll leave this issue open.

Two interim suggestions:

1. You can of course build your own docker image off any base image that you choose.
2. Dockerhub's docker image may be an appropriate community-supported image.

[akauppi]

Based on this the current base seems to be Ubuntu 20.04.

I was trying to use it today, but the >800MB size (!!) threw me off.

[haroonc]

Fixed by #836