[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

• build/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	Issue	Exploit Maturity
	Integer Overflow or Wraparound SNYK-UBUNTU2004-PCRE3-583594	No Known Exploit
	Improper Verification of Cryptographic Signature SNYK-UBUNTU2004-PERL-1930907	No Known Exploit
	Time-of-check Time-of-use (TOCTOU) SNYK-UBUNTU2004-SHADOW-577863	No Known Exploit
	Time-of-check Time-of-use (TOCTOU) SNYK-UBUNTU2004-SHADOW-577863	No Known Exploit
	Out-of-bounds Read SNYK-UBUNTU2004-TAR-1126791	No Known Exploit

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