Change Docker container image to 20.04 LTS

[ashao]

Github Actions fail due to the docker build step relying on
Ubuntu 21.04. This version of Ubuntu has reached end of life and
so the package install step of the workflow was failing. The rest
of the testing for SmartRedis points to 20.04 and so both the
tests and Dockerfile is synced.

Closes #274

[codecov-commenter]

Codecov Report

Merging #276 (cd2864f) into develop (538bb90) will not change coverage.
The diff coverage is n/a.

❗ Current head cd2864f differs from pull request most recent head 2c004f7. Consider uploading reports for the commit 2c004f7 to get more accurate results

@@           Coverage Diff            @@
##           develop     #276   +/-   ##
========================================
  Coverage    80.16%   80.16%           
========================================
  Files           56       56           
  Lines         3444     3444           
========================================
  Hits          2761     2761           
  Misses         683      683