Add apt publisher for Ubuntu 20.04.

apt-publisher/Dockerfile-ubuntu2004

+FROM ubuntu:20.04
+
+# based on https://www.digitalocean.com/community/tutorials/how-to-use-reprepro-for-a-secure-package-repository-on-ubuntu-14-04
+
+MAINTAINER Ray Burgemeestre
+
+RUN apt-get update && \
+    apt install -y rng-tools gnupg2 \
+                   apache2 dpkg-dev dpkg-sig \
+                   apt-utils reprepro && \
+    \
+    mkdir -p /repo && \
+    mkdir -p ~/.gnupg && \
+    rm -rf /var/lib/apt/lists/*
+
+CMD "/bin/bash"

apt-publisher/Makefile

 	        \
 			    packages/
 
-docker:
-	docker build . -t rayburgemeestre/apt-ubuntu:18.04 -f Dockerfile
+docker-ubuntu1804:
+	docker build . -t rayburgemeestre/apt-ubuntu:18.04 -f Dockerfile-ubuntu1804
+
+docker-ubuntu2004:
+	docker build . -t rayburgemeestre/apt-ubuntu:20.04 -f Dockerfile-ubuntu2004
 
 run-initialize-once:
 	echo Please refer to README. Initial steps were done once manually and tar of artifacts was zipped.