docs: Image Digest Pinning

[adambkaplan]

Description of the change:

Add an "advanced" article to describe how relatedImages and image digest pinning works. This includes the use of RELATED_IMAGE_* environment variables to digest pin operands deployed by the operator, which is not currently documented.

Motivation for the change:

I was very confused by the behavior of relatedImages and digest pinning. For example - when attempting to set relatedImages in the base CSV manifest, I discovered that operator-sdk generate bundle would always overwrite the relatedImages field in tree. It wasn't until I started iterating on a pull request (#6869) that I discovered there is an undocumented environment variable that lets operator authors define and wire in image digests for their operands.

Checklist

If the pull request includes user-facing changes, extra documentation is required:

• Add a new changelog fragment in changelog/fragments (see changelog/fragments/00-template.yaml)
• Add or update relevant sections of the docs website in website/content/en/docs

[openshift-bot]

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[openshift-bot]

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[adambkaplan]

@sdk-approvers is operator-sdk accepting documentation improvements?

[openshift-bot]

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[openshift-ci]

@openshift-bot: Closed this PR.

Details

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