[Snyk] Security upgrade debian from 9.5-slim to stretch-20210621-slim#16

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snyk-bot wants to merge 1 commit intomainfrom snyk-fix-2e7885ba99fc9a3d109f99c4da9ecaaa

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Commits on Jul 15, 2021

fix: content/actions/creating-actions/dockerfile-support-for-github-actions.md to reduce vulnerabilities
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