[Snyk] Security upgrade debian from 9.5-slim to stretch-20210621-slim#49

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snyk-fix-4abd49a45a08e1a9fb7cd60da7f9435a

Commits on Jun 30, 2021

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