Bug 1840: 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 1840

Change-Id: I2cf79d9e697ba1267e2a8d00a180412a2887e1bd
Signed-off-by: beierlm <mark.beierl@canonical.com>