[Snyk] Security upgrade python from 3.8-alpine3.12 to rc-alpine3.12

[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

• cmak2zk/Dockerfile

We recommend upgrading to python:rc-alpine3.12, as this image has only 24 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
	571	Integer Overflow or Wraparound SNYK-ALPINE312-EXPAT-2342150	No Known Exploit
	567	Integer Overflow or Wraparound SNYK-ALPINE312-EXPAT-2342151	No Known Exploit
	571	Integer Overflow or Wraparound SNYK-ALPINE312-EXPAT-2342157	No Known Exploit
	571	Integer Overflow or Wraparound SNYK-ALPINE312-EXPAT-2342158	No Known Exploit
	500	Buffer Overflow SNYK-ALPINE312-OPENSSL-1569452	No Known Exploit

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[krux-ci]

Can one of the admins verify this patch?