[Snyk] Security upgrade ubuntu from focal to 20.04

[timfeirg]

This PR was automatically created by Snyk using the credentials of a real user.

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 12 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
	364	Improper Authentication SNYK-UBUNTU2004-SHADOW-5879186	No Known Exploit
	364	Improper Authentication SNYK-UBUNTU2004-SHADOW-5879186	No Known Exploit
	321	CVE-2023-26604 SNYK-UBUNTU2004-SYSTEMD-3339226	Mature
	321	CVE-2023-26604 SNYK-UBUNTU2004-SYSTEMD-3339226	Mature
	300	CVE-2020-22916 SNYK-UBUNTU2004-XZUTILS-5854646	No Known Exploit

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