[Snyk] Security upgrade ubuntu from 20.04 to groovy-20210325#21

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snyk-bot wants to merge 1 commit intomasterfrom snyk-fix-b95f7d04b4e97c787f418f4a0d51133a

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Commits on Apr 13, 2021

fix: dffml/skel/operations/Dockerfile to reduce vulnerabilities
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