Bug 1850: 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 1850

Change-Id: I29dcb5862b26021cecb4e581bd3cbd870682c968
Signed-off-by: beierlm <mark.beierl@canonical.com>