[dlor]

That's a bit out of date - today layer IDs are for the most part sha256's of the contents of the layer tar.gz.

However, it's still very difficult to generate the same layer ID twice. Timestamps permeate the layer contents itself in the form of file mtimes. The final image metadata itself also contains several timestamps.

Docker has a local cache of layers that helps simulate reproducibility. But if you clear that cache or use a different build machine, you will have a very hard time ever generating the same layer ID again.