remove deprecated line

Dockerfile

@@ -2,11 +2,8 @@ FROM ubuntu:20.04
 
 LABEL Maintainer="James Hiebert <[email protected]>"
 
-# WARNING: This should be removed with the introduction of 20.04
-RUN sed -i 's/archive.ubuntu.com/old-releases.ubuntu.com/g;s/security.ubuntu.com/old-releases.ubuntu.com/g' /etc/apt/sources.list
-
 # temporarily commenting this out - it is failing for some reason.
-# RUN apt-get update
+RUN apt-get update
 RUN apt-get -yq install libhdf5-dev libnetcdf-dev libgdal-dev
 RUN apt-get -yq install libyaml-dev python python-dev python-pip
 RUN apt-get -yq install python-virtualenv cython