[Snyk] Security upgrade node from 12.18.4-buster to 12-buster

[jbowes]

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 node:12-buster, as this image has only 439 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
	714	Buffer Overflow SNYK-DEBIAN10-OPENSSL-1569403	No Known Exploit
	614	Loop with Unreachable Exit Condition ('Infinite Loop') SNYK-DEBIAN10-OPENSSL-2426310	No Known Exploit
	714	Buffer Overflow SNYK-DEBIAN10-PYTHON37-1063182	No Known Exploit
	614	NULL Pointer Dereference SNYK-DEBIAN10-SUBVERSION-1071814	No Known Exploit
	614	Use After Free SNYK-DEBIAN10-SUBVERSION-2635642	No Known Exploit

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