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

[snyk-bot]

Keeping your Docker base image up-to-date means you’ll benefit from security fixes in the latest version of your chosen image.

Changes included in this PR

• content/actions/creating-actions/dockerfile-support-for-github-actions.md

We recommend upgrading to debian:stretch-slim, as this image has only 84 known vulnerabilities. To do this, merge this pull request, then verify your application still works as expected.

Some of the most important vulnerabilities in your base image include:

Severity	Priority Score / 1000	Issue	Exploit Maturity
	614	CVE-2019-3462 SNYK-DEBIAN9-APT-407402	No Known Exploit
	514	Out-of-bounds Write SNYK-DEBIAN9-E2FSPROGS-468975	No Known Exploit
	514	Out-of-bounds Write SNYK-DEBIAN9-E2FSPROGS-540843	No Known Exploit
	514	Use of a Broken or Risky Cryptographic Algorithm SNYK-DEBIAN9-LIBGCRYPT20-1582895	No Known Exploit
	514	Loop with Unreachable Exit Condition ('Infinite Loop') SNYK-DEBIAN9-TAR-312293	No Known Exploit

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[github-actions]

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The table contains an overview of files in the content directory that have been changed in this pull request. It's provided to make it easy to review your changes on the staging site. Please note that changes to the data directory will not show up in this table.

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Content directory changes

You may find it useful to copy this table into the pull request summary. There you can edit it to share links to important articles or changes and to give a high-level overview of how the changes in your pull request support the overall goals of the pull request.

Article in Staging	Live Article	Source	Changes
dockerfile-support-for-github-actions	dockerfile-support-for-github-actions