dockerfile: add (ADD|COPY) --timestamp=<RFC3339Nano>

[AkihiroSuda]

(ADD|COPY) --timestamp=<RFC3339Nano> would be useful for more reproducible builds.

e.g.,

FROM busybox AS base
RUN echo hello >/hello

FROM scratch
COPY --from=base --timestamp="2006-01-02T15:04:05Z" /hello /hello

Closes #2910

[tonistiigi]

@AkihiroSuda Do you have more use cases in mind than reproducible builds. I've been working on SOURCE_DATE_EPOCH support in master...tonistiigi:source-date-epoch . Eventually, if this is set it should set timestamps for all copies in Dockerfile automatically. Currently, it is a build arg but if we want it to be set in Dockerfile as well, then could be a build directive, #epoch= etc.

[tonistiigi]

Also, looks like there is a related bug in LLB layer that needs to be fixed as well for this #2884

[AkihiroSuda]

#epoch=

SGTM if it applies to http and local source ops as well as to exporters

[hanikesn]

I'd love to revisit this idea. I currently worked on making our builds reproducible and timestamps of files were a major issue. Unfortunately I can't use rewrite-timestamp as it's way quite slow for interactive builds as the rewrite step is not cached at the respective layers.

• Avoid applying SOURCE_DATE_EPOCH to base images #4663

Fixes the problem only partially as we have a quite big monolithic Dockerfile (thanks to bake ❤️). And we mostly have 1-2 build steps in the final image were the timestamps matter. Now that I'm writing this I'm wondering whether applying rewrite-timestamp at the layer level so it's cached would be the better approach (at least usability wise).