docker: Add tools/docker/data/ssh.keys/README.txt

[anomiex]

Proposed changes:

The main purpose of this is to exist to work around #31498 absent an upstream fix.

Note every line in the file needs to begin with # or it'll break the container in a different way.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
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Jetpack product discussion

Fixes #31498

Does this pull request change what data or activity we track or use?

No

Testing instructions:

• Without any key files in tools/docker/data/ssh.keys/, running jetpack docker up should not have the sftp container exit with the error in the linked issue.
  • Note you may need to docker container rm jetpack_dev_sftp_1 or the like to clean up an old broken version of the sftp container.

[github-actions]

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

• ✅ Include a description of your PR changes.
  
• ✅ All commits were linted before commit.
  
• ✅ Add a "[Status]" label (In Progress, Needs Team Review, ...).
  
• ✅ Add testing instructions.
  
• ✅ Specify whether this PR includes any changes to data or privacy.
  
• ✅ Add changelog entries to affected projects
  

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