Bug 1849: 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 1849

Change-Id: I8a589bbc76bb11f4bc0c42ebb727729da5c5c906
Signed-off-by: beierlm <mark.beierl@canonical.com>