[Snyk] Security upgrade python from 3-slim-stretch to 3.7.11-slim-stretch

[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

• _action_files/fastpages-nbdev.Dockerfile

We recommend upgrading to python:3.7.11-slim-stretch, as this image has only 117 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	Integer Overflow or Wraparound SNYK-DEBIAN9-OPENSSL-1075328	No Known Exploit
	614	Out-of-bounds Read SNYK-DEBIAN9-OPENSSL-1569399	No Known Exploit
	614	Loop with Unreachable Exit Condition ('Infinite Loop') SNYK-DEBIAN9-OPENSSL-2426305	No Known Exploit
	778	OS Command Injection SNYK-DEBIAN9-OPENSSL-2807589	No Known Exploit
	514	Loop with Unreachable Exit Condition ('Infinite Loop') SNYK-DEBIAN9-TAR-312293	No Known Exploit

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[changeset-bot]

⚠️ No Changeset found

Latest commit: ff7ede3

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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