functions-infrastructure-as-code.md: missing DOCKER_REGISTRY_SERVER_* settings leads to HTTP/500

[clook]

Add more information about unexpected and undocumented behaviour if DOCKER_REGISTRY_SERVER_* settings are missing for a private repo reference.

[prmerger-automator]

@clook : Thanks for your contribution! The author(s) have been notified to review your proposed change.

[clook]

It took me more than one hour to understand this 500 error. Even if the must-have is documented, the behavior when it is not followed is not clear.

[learn-build-service-prod]

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[Court72]

@ggailey777

Can you review the proposed changes?

Important: When the changes are ready for publication, adding a #sign-off comment is the best way to signal that the PR is ready for the review team to merge.

#label:"aq-pr-triaged"
@MicrosoftDocs/public-repo-pr-review-team

[ggailey777]

Thanks @clook, but I'm going to move your info a different place (and fix the link right there that would have been helpful if it was working. I'll grab your commit so you get credit for helping out 😄