misc: Added Dockerfile for Ubuntu 20.04

misc: Added Dockerfile for Ubuntu 20.04

This Dockerfile creates an image which simulates an Ubuntu 20.04
environment. Unlike the Ubuntu 18.04 Dockerfile, this does not use
Python2. It uses exclusively Python3. Ubuntu 20.04 has Python3 installed
by default. The image this Dockerfile creates can be obtained from
"gcr.io/gem5-test/ubuntu-20.04_all-dependencies". To pull:

docker pull gcr.io/gem5-test/ubuntu-20.04_all-dependencies

Change-Id: I73b51028e0d6a3198aa6e7b1906d20ed6eb6c815
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28889
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>