Bug 1842: LTS Support

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

Fixes Bug 1842

Change-Id: Iefa3c72fe200ecf4f0b4081db76d203ac38a50e7
Signed-off-by: beierlm <mark.beierl@canonical.com>