[Snyk] Security upgrade debian from 9.5-slim to stretch-20210902-slim#99

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snyk-bot wants to merge 1 commit intomainfrom snyk-fix-b99dc97c895ec8da135d8d259ea567a0

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Commits on Sep 28, 2021

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