[Snyk] Security upgrade debian from 9.5-slim to 9-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:9-slim, as this image has only 86 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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