Bug 1844: 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 1844

Change-Id: Iccea9a5121fa8e701ae18c64d71d094ee7c535c7
Signed-off-by: beierlm <mark.beierl@canonical.com>