[Snyk] Security upgrade debian from 9.5-slim to stretch-20210621-slim#30

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snyk-bot wants to merge 1 commit intomainfrom snyk-fix-7bcc5dbdf3354168717d05d0159d27c5

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Commits on Jun 30, 2021

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