[Snyk] Security upgrade ubuntu from latest to 20.04

[snyk-bot]

Keeping your Docker base image up-to-date means you’ll benefit from security fixes in the latest version of your chosen image.

Changes included in this PR

• Dockerfile

We recommend upgrading to ubuntu:20.04, as this image has only 14 known vulnerabilities. To do this, merge this pull request, then verify your application still works as expected.

Some of the most important vulnerabilities in your base image include:

Severity	Priority Score / 1000	Issue	Exploit Maturity
	321	Improper Check for Dropped Privileges SNYK-UBUNTU2004-BASH-581100	Mature
	364	Information Exposure SNYK-UBUNTU2004-LIBGCRYPT20-1297919	No Known Exploit
	514	Use of a Broken or Risky Cryptographic Algorithm SNYK-UBUNTU2004-LIBGCRYPT20-1583851	No Known Exploit
	150	Time-of-check Time-of-use (TOCTOU) SNYK-UBUNTU2004-SHADOW-577863	No Known Exploit
	150	Time-of-check Time-of-use (TOCTOU) SNYK-UBUNTU2004-SHADOW-577863	No Known Exploit

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[codecov]

Codecov Report

Merging #304 (f6ac4d9) into main (57b7d17) will not change coverage.
The diff coverage is n/a.

❗ Current head f6ac4d9 differs from pull request most recent head d029568. Consider uploading reports for the commit d029568 to get more accurate results

@@           Coverage Diff           @@
##             main     #304   +/-   ##
=======================================
  Coverage   35.11%   35.11%           
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  Files          44       44           
  Lines        2862     2862           
=======================================
  Hits         1005     1005           
  Misses       1739     1739           
  Partials      118      118