update best practices

[adyanth]

Description

Update the ADD recommendation to be clear on the behavior over RUN curl and counterparts.

Related issues or tickets

Closes https://github.com/docker/docs/issues/22168 Related to https://github.com/docker/docs/pull/18582

Reviews

Initially changed as a result of discussion in https://github.com/moby/buildkit/issues/4125

• [X] Technical review @dvdksn @tonistiigi @crazy-max @thaJeztah
• [ ] Editorial review
• [ ] Product review

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[docker-robot[bot]]

Thanks for the pull request. We'd like to make our product docs better, but haven’t been able to review all the suggestions. As our docs have also diverged, we do not have the bandwidth to review and rebase old pull requests.

If the updates are still relevant, review our contribution guidelines and rebase your pull request against the latest version of the docs, then mark it as fresh with a /remove-lifecycle stale comment. If not, this pull request will be closed in 30 days. This helps our maintainers focus on the active pull requests.

Prevent pull requests from auto-closing with a /lifecycle frozen comment.

/lifecycle stale