Set ubuntu 20.04 as new base image

We would at some point need to set this as the default.
To prepare for that, I think we should make this an option
that is available, and run our tests on this too.

The R stack seems possibly currently unavailable for 20.04
(see https://community.rstudio.com/t/rstudio-server-ubuntu-20-04/64188)
but I'll try it out

repo2docker/buildpacks/base.py

@@ -14,7 +14,7 @@
 from traitlets import Dict
 
 TEMPLATE = r"""
-FROM buildpack-deps:bionic
+FROM buildpack-deps:focal
 
 # avoid prompts from apt
 ENV DEBIAN_FRONTEND=noninteractive
@@ -55,7 +55,7 @@
  ${NB_USER}
 
 RUN wget --quiet -O - https://deb.nodesource.com/gpgkey/nodesource.gpg.key | apt-key add - && \
- DISTRO="bionic" && \
+ DISTRO="focal" && \
  echo "deb https://deb.nodesource.com/node_10.x $DISTRO main" >> /etc/apt/sources.list.d/nodesource.list && \
  echo "deb-src https://deb.nodesource.com/node_10.x $DISTRO main" >> /etc/apt/sources.list.d/nodesource.list
 

tests/memlimit/dockerfile/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:bionic
+FROM ubuntu:focal
 
 RUN apt-get update && apt-get install --yes python3