Closes #1644

What has been done to verify that this works as intended?

Ran CI a lot.

Why is this the best possible solution? Were any other approaches considered?

Per #1644 (comment):

This with --iidfile is the documented expected behavior that scripts can rely on that we shouldn't break, as opposed to pattern matching from the output.

• [RFC] Add some output to logs for backward compat moby/buildkit#1155 (comment)

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

No effect.

Does this change require updates to documentation? If so, please file an issue here and include the link below.

No.

Before submitting this PR, please make sure you have:

• branched off and targeted the next branch OR only changed documentation/infrastructure (master is stable and used in production)
• verified that any code or assets from external sources are properly credited in comments or that everything is internally sourced