[Snyk] Security upgrade ubuntu from 16.04 to xenial-20210416

[scatools-demo]

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

• tensorflow/tools/ci_build/Dockerfile.cpu

We recommend upgrading to ubuntu:xenial-20210416, as this image has only 81 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
	536	Improper Check for Dropped Privileges SNYK-UBUNTU1604-BASH-542609	Mature
	514	Directory Traversal SNYK-UBUNTU1604-DPKG-2848000	No Known Exploit
	514	Out-of-bounds Read SNYK-UBUNTU1604-E2FSPROGS-2770734	No Known Exploit
	614	Allocation of Resources Without Limits or Throttling SNYK-UBUNTU1604-SYSTEMD-1320131	No Known Exploit
	514	Out-of-bounds Read SNYK-UBUNTU1604-TAR-3261141	No Known Exploit

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🦉 Allocation of Resources Without Limits or Throttling
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