[Snyk] Security upgrade debian from stretch-20210329-slim to stretch-20210721-slim#124

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snyk-fix-4d8e2e0a8d50cc1a16c1dfd6c17dc8e5

Commits on Jul 23, 2021

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