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>
diff --git a/requirements-dist.txt b/requirements-dist.txt
index 77debcf..8192b48 100644
--- a/requirements-dist.txt
+++ b/requirements-dist.txt
@@ -20,4 +20,7 @@
     # via -r requirements-dist.in
 
 # The following packages are considered to be unsafe in a requirements file:
-# setuptools
+setuptools==59.8.0
+    # via
+    #   -r requirements-dist.in
+    #   setuptools-version-command