Bug 1841: 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 1841

Change-Id: I70af3cfab7484d7b284c143d554bef9f0be51812
Signed-off-by: beierlm <mark.beierl@canonical.com>
diff --git a/Dockerfile b/Dockerfile
index a3f7400..931da3e 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -26,6 +26,12 @@
 
 FROM ubuntu:18.04
 
+ARG APT_PROXY
+RUN if [ ! -z $APT_PROXY ] ; then \
+    echo "Acquire::http::Proxy \"$APT_PROXY\";" > /etc/apt/apt.conf.d/proxy.conf ;\
+    echo "Acquire::https::Proxy \"$APT_PROXY\";" >> /etc/apt/apt.conf.d/proxy.conf ;\
+    fi
+
 RUN DEBIAN_FRONTEND=noninteractive apt-get update && \
     DEBIAN_FRONTEND=noninteractive apt-get -y install \
         debhelper \