[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

• dev/Dockerfile.cron

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
	Improper Check for Dropped Privileges SNYK-UBUNTU2004-BASH-581100	Mature
	Information Exposure SNYK-UBUNTU2004-LIBGCRYPT20-1297919	No Known Exploit
	Use of a Broken or Risky Cryptographic Algorithm SNYK-UBUNTU2004-LIBGCRYPT20-1583851	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

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