Make it easier for image layers to overlap

[jonathan-boudreau-work]

Use case

On pull requests, we build images and push them to a registry. This is done to deploy to an environment temporarily. Unfortunately, the images are quite large, but bulk of the content doesn't change much.

We solved this issue by making the layers reproducible. If the blob is the same, then it will just be skipped when we upload the image, resulting in lower bandwidth / storage costs.

Currently this means having to use multi-stage builds to overwrite the file timestamps with touch -t all over the place. Its quite involved, and I think docker hub could benefit greatly from the reduced storage costs from something like this.

Proposal

Since this could be viewed as backward incompatible, a buildx build flag which automatically zeroes out the timestamps of files COPY'ed or ADD'ed into the image could be implemented to make it easier to have reproducible layer builds.

[tonistiigi]

This is tracked in moby/buildkit#1058 and partially implemented in moby/buildkit#2918

You can also do this by using previous builds as a cache source with --cache-from. Then the layers are reused directly.