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: I1f64fa5364d909e6526d38395d4d2c1cc25f6f7d
Signed-off-by: beierlm <mark.beierl@canonical.com>