Bug 1863: LTS Support

Updates base to Ubuntu 20.04
Adds apt cache to stage 2 dockerfile
Updates python to 3.8
Updates pip dependencies to more recent versions

Fixes bug 1863

Change-Id: If82a6f3169f7e800c6284c7f3e7f232b7fc73ea5
Signed-off-by: beierlm <mark.beierl@canonical.com>