Upgrade base image to fix issue with get-pip.py

[tanertopal]

Currently all cubes will break when trying to install pip using get-pip.py. The reason for this is that get-pip.py has dropped compatibility with Python 3.6 (see here) which is default installed version of the python3 package in Ubuntu 18.04. Using the Ubuntu 20.04 base image will resolve this issue.

The error which will otherwise be thrown in the docker build process is:

 => ERROR [3/6] RUN curl -fSsL -O https://bootstrap.pypa.io/get-pip.py &&     python3 get-pip.py &&     rm get-pip.py                                                                            0.8s
------
 > [3/6] RUN curl -fSsL -O https://bootstrap.pypa.io/get-pip.py &&     python3 get-pip.py &&     rm get-pip.py:
#6 0.753 ERROR: This script does not work on Python 3.6 The minimum supported Python version is 3.7. Please use https://bootstrap.pypa.io/pip/3.6/get-pip.py instead.

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