[Docker] Upgrade base image

Summary

The current base image is FROM ubuntu:16.04 which is already a few years old.

Proposal

I propose to upgrade the base image to one of the following:

ubuntu:18.04

This is the most obvious variant, has upstream support and does not require that many changes.

debian:buster-slim

This base image (especially the slim variant) is slightly smaller that the current Ubuntu image (#1712 (comment 20515578)). Most other container in the Docker library are based on Debian. Since the layers can be cached and shared across different images, this could save additional resources on hosts where multiple containers are running in parallel.