docker: use ubuntu 18.04 as base?

Currently we have it as 19.10. Some relevant info:

• 19.10 can call pip3 rather than python3 -m pip in 18.04
• 19.10 has Python 3.7 (18.04 ships 3.6).
• 18.04 is a LTS, 19.10 is not.

Or we could use some other base image such as python:3.8 or fedora:32 or ...