`COPY --rewrite-timestamp ...` to apply SOURCE_DATE_EPOCH build arg value to the timestamps of the files

Description

Hi! Trying to produce reproducible layers, I realize that COPYing files from some layer into a scratch final stage does not respect SOURCE_DATE_EPOCH build arg.

For example, I'm having to do something like:

FROM ... AS some-stage
RUN --mount=from=some-previous-stage,dst=/bla ... >/this
ARG SOURCE_DATE_EPOCH
RUN find /this -print0 | xargs -0 touch --no-dereference --date=@$SOURCE_DATE_EPOCH

FROM scratch AS some-layer
COPY --link --from=some-stage /this /there

Note: I am using this "run --mount then copy --from" pattern many many times until a final scratch stage. Also, I am using --cache-from/to with type=registry,mode=max and --output=type=tar. This is buildx notation but you get the idea.

First: am I doing something wrong or unexpected? Checking with diffoci it seems I really do have to touch files to get a quiet diff.

I would like to extend on today's implementation of the build arg and the dockerfile frontend regarding rewrite-timestamp.
I propose that, given the build arg set, COPY should support a new flag which will touch all files added in this stage.

FROM ... AS some-stage
RUN --mount=from=some-previous-stage,dst=/bla ... >/this

FROM scratch AS some-layer
COPY --link --from=some-stage --rewrite-timestamp /this /there

Per https://github.com/moby/buildkit/blob/202e28fe031a3be7eba17fb4382e4bbb0acf69b3/docs/build-repro.md#source_date_epoch rewrite-timestamp only affects the image exporter.

cc https://github.com/moby/buildkit/pull/2911 https://github.com/moby/buildkit/issues/3973#issuecomment-3318117271

Read the SOURCE_DATE_EPOCH and rewrite-timestamp discussion in docs/build-repro.md, then trace the Dockerfile frontend and image exporter mentioned in the issue. Determine how a new COPY flag should apply the build-arg timestamp to files added to a stage, and validate that the resulting layers are reproducible without the manual touch step.