update docker images and submodule workflow (was drop repos.conf)

[rwalker-apple]

Problem

• git 2.17 (default on ubuntu:bionic)does not support git for github action-based wokflows
• repos.conf and Makefile-bootstrap are deprecated in favor of git submodules (but are required for the automake-based "distcheck" target to function)

Summary of Changes

• make repos.conf a symbolic link to .gitmodules, so there's one source of truth for submodules
• update docker images to be based on ubuntu:focal, which brings current cmake, clang, openssl, and git (which allows github workflows to run on a git repository, and get git submodules)
• update docs

[andy31415]

This also have quite some docker file updates - unsure if PR description actually captures those changes description.

[andy31415]

This also have quite some docker file updates - unsure if PR description actually captures those changes description.

@rwalker-apple I believe a description 'fail to drop repos.conf' may need to be updated if we want to include the whole PR since the change of 'update docker to new ubuntu image' seems to be a important change. Did not yet merge the PR for this reason.