Bug 1847: 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 1847

Change-Id: Icbe9e9efc0edd957a4b8c0c461cf9d32c716df3e
Signed-off-by: beierlm <mark.beierl@canonical.com>
diff --git a/requirements.txt b/requirements.txt
index 6f8103b..8b8b3fa 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,11 +1,3 @@
-jinja2==2.11.3
-    # via -r requirements.in
-markupsafe==1.1.1
-    # via jinja2
-pymzn==0.18.3
-    # via -r requirements.in
-pyyaml==5.4.1
-    # via -r requirements.in
 #######################################################################################
 # Copyright ETSI Contributors and Others.
 #
@@ -22,3 +14,11 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 #######################################################################################
+jinja2==3.0.3
+    # via -r requirements.in
+markupsafe==2.0.1
+    # via jinja2
+pymzn==0.18.3
+    # via -r requirements.in
+pyyaml==5.4.1
+    # via -r requirements.in