Bug 1848: 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 1848

Change-Id: I203d2d3f9ba636dfc12e0a640bdf445529713402
Signed-off-by: beierlm <mark.beierl@canonical.com>