Upgrade release Dockerfile from Ubuntu 16.04 to 20.04 (#1631)

Author: bergmeister

tools/docker/ubuntu16/Dockerfile renamed to tools/docker/ubuntu/Dockerfile

@@ -1,9 +1,9 @@
-FROM mcr.microsoft.com/powershell:ubuntu-16.04
+FROM mcr.microsoft.com/powershell:ubuntu-20.04
 
 ENV __InContainer 1
 
 RUN apt update -qq && apt install -q -y wget git apt-transport-https
-RUN wget -q https://packages.microsoft.com/config/ubuntu/16.04/packages-microsoft-prod.deb && dpkg -i packages-microsoft-prod.deb
+RUN wget -q https://packages.microsoft.com/config/ubuntu/20.04/packages-microsoft-prod.deb && dpkg -i packages-microsoft-prod.deb
 
 RUN apt update -qq && \
     cd / && \