Bug 1841: 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 1841

Change-Id: I70af3cfab7484d7b284c143d554bef9f0be51812
Signed-off-by: beierlm <mark.beierl@canonical.com>