[docker] Switch base image from alpine:3.14 to ubuntu:20.04

Summary: This is Ubuntu "minimal" or whatever according to: https://ubuntu.com/blog/minimal-ubuntu-released

Test Plan: Putting this up to see what happens with the CI

Reviewers: varun, geekbrother, karol-bisztyga, jimpo, ashoat

Reviewed By: varun, karol-bisztyga, ashoat

Subscribers: benschac, ashoat, palys-swm, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3212