Bug 1842: 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 1842

Change-Id: I1f64fa5364d909e6526d38395d4d2c1cc25f6f7d
Signed-off-by: beierlm <mark.beierl@canonical.com>
diff --git a/requirements-dist.txt b/requirements-dist.txt
index 15a902a..8192b48 100644
--- a/requirements-dist.txt
+++ b/requirements-dist.txt
@@ -1,13 +1,3 @@
-setuptools-version-command==99.9
-    # via -r requirements-dist.in
-stdeb==0.10.0
-    # via -r requirements-dist.in
-
-# The following packages are considered to be unsafe in a requirements file:
-setuptools==59.8.0
-    # via
-    #   -r requirements-dist.in
-    #   setuptools-version-command
 #######################################################################################
 # Copyright ETSI Contributors and Others.
 #
@@ -24,3 +14,13 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 #######################################################################################
+setuptools-version-command==99.9
+    # via -r requirements-dist.in
+stdeb==0.10.0
+    # via -r requirements-dist.in
+
+# The following packages are considered to be unsafe in a requirements file:
+setuptools==59.8.0
+    # via
+    #   -r requirements-dist.in
+    #   setuptools-version-command