feat: Update to Ubuntu 20.04 base image (matthewfeickert#9)

* Update to ubuntu:20.04 as the base image
   - c.f. https://wiki.ubuntu.com/FocalFossa/ReleaseNotes/20.04

Author: matthewfeickert

Dockerfile

@@ -1,6 +1,4 @@
-FROM ubuntu:bionic
-
-MAINTAINER Matthew Feickert <[email protected]>
+FROM ubuntu:20.04
 
 USER root
 WORKDIR /root
@@ -42,7 +40,7 @@ RUN apt-get -qq -y update && \
     mv /usr/bin/lsb_release /usr/bin/lsb_release.bak && \
     apt-get -y autoclean && \
     apt-get -y autoremove && \
-    rm -rf /var/lib/apt-get/lists/*
+    rm -rf /var/lib/apt/lists/*
 
 COPY install_python.sh install_python.sh
 RUN bash install_python.sh ${PYTHON_VERSION_TAG} ${LINK_PYTHON_TO_PYTHON3} && \

README.md

@@ -1,6 +1,6 @@
 # Python3 on Ubuntu Docker
 
-Dockerfile for image built off [Ubuntu 18.04](https://wiki.ubuntu.com/BionicBeaver/ReleaseNotes/18.04) containing [Python 3.8](https://www.python.org/downloads/release/python-381/) ([Python 3.6](https://www.python.org/downloads/release/python-368/), [Python 3.7](https://www.python.org/downloads/release/python-374/)) built from source
+Dockerfile for image built off [Ubuntu 20.04](https://wiki.ubuntu.com/FocalFossa/ReleaseNotes/20.04) containing [Python 3.8](https://www.python.org/downloads/release/python-381/) ([Python 3.6](https://www.python.org/downloads/release/python-368/), [Python 3.7](https://www.python.org/downloads/release/python-374/)) built from source
 
 [![Docker Automated build](https://img.shields.io/docker/automated/matthewfeickert/docker-python3-ubuntu.svg)](https://hub.docker.com/r/matthewfeickert/docker-python3-ubuntu/)
 [![Docker Build Status](https://img.shields.io/docker/build/matthewfeickert/docker-python3-ubuntu.svg)](https://hub.docker.com/r/matthewfeickert/docker-python3-ubuntu/builds/)