Bug 1846: 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 1846

Change-Id: I589199979818b0fe5892df1d40849da8f6e06ded
Signed-off-by: beierlm <mark.beierl@canonical.com>