documentation: Add information on pinning images by digest.
What does this MR do?
In documentation for .gitlab-ci.yml, add an example of pinning images by digest. This is considered best practice in some situations, but AFAICT, is not shown in the Gitlab docs.
Author's checklist
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Follow the Documentation Guidelines and Style Guide. - [-] If applicable, update the permissions table.
- [-] Link docs to and from the higher-level index page, plus other related docs where helpful.
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Apply the documentation label.
Review checklist
All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.
1. Primary Reviewer
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Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.
2. Technical Writer
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3. Maintainer
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Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review. -
Ensure a release milestone is set. -
If there has not been a technical writer review, create an issue for one using the Doc Review template.