Although it’s fine and nice to have an image digest in the manifest, this is not a solution for the points made above, and the reason was already hinted above. We can look at it from this perspective: we might also create a digest from your machine at the time you built the snap. Okay, now we know that environment was the same across several builds, but what was on the actual environment when it was built? That’s what the manifest is supposed to answer.
And again, yes, the fact it’s a cleanbuild improves the situation a bit because it’s a shared resource, but it’s still a similar feeling: in 6 months that image will most likely be different, and your digest may not even be around anymore, and we’ll have no idea about what was actually used. Even if the image is around, it would be much better to have data in the manifest which properly hint about what was used, instead of a digest.