Simplify image-building initialization script
Simplify image-building initialization script

* Remove installing software-properties-common since our sources.list is
  quite simple, and so we do not need add-apt-repository for now.
* Replace the full python3.8 package with python3-minimal, which also
  installs Python 3.8.

clchiou/garage,clchiou/garage,clchiou/garage,clchiou/garage