[Snyk] Security upgrade debian from 9.5-slim to stretch-20210621-slim#19

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snyk-bot wants to merge 1 commit intomainfrom snyk-fix-24406604d1313aed7eebe1065a66a1d4

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Commits on Jul 15, 2021

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