Bug 1843: 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 1843

Change-Id: Iec7c73cb99f5ed3f0e07666d8c8c8be8ca91c0ea
Signed-off-by: beierlm <mark.beierl@canonical.com>