PRs updates may cause unwanted PR creation in `docker-library/official-images`

Situation

When Dockerfile-template.* changes are made and Dockerfile updates in the release line directories are also made, then the repo automatically triggers a PR to build a new release. The workflow official-pr is responsible for this.

Unless the update is part of a new release, then Dockerfile updates should generally not be done manually. These updates should be left to the automated processes that are triggered when the repo's automatic-updates.yml polling workflow detects a new Node.js release.

Suggestion

Research the background and add instructions to the CONTRIBUTING document about when Dockerfile updates should be made directly, and when it should be left to automation.

For those cases where no direct Dockerfile update should be included in a PR, describe also how to test such PRs.

Read CONTRIBUTING.md alongside .github/workflows/official-pr.yml and automatic-updates.yml to understand when Dockerfile changes trigger official-image pull requests. Update CONTRIBUTING.md with the direct-update exceptions and instructions for testing PRs that should not include a Dockerfile update; done means both cases are clearly documented.