You actually can sort of do this just inside a modern docker file, with COPY --link
That version of copy overlays the copied content without any knowledge of its underlay, so if the underlay changes, the hash of the layer created by COPY remains identical.
Obviously there are lots of situations this doesn't work for, but say you have a pool of Ubuntu packages (the system layer) and then you separately copy a CMake installspace or built uv project from a build stage, that project layer can be identical in hash if it's copied onto an updated system layer.
In practice, it's hard to get this to work because it's very easy in docker to accidentally trigger a rebuild for some random reason, but all the pieces are there.
That version of copy overlays the copied content without any knowledge of its underlay, so if the underlay changes, the hash of the layer created by COPY remains identical.
Obviously there are lots of situations this doesn't work for, but say you have a pool of Ubuntu packages (the system layer) and then you separately copy a CMake installspace or built uv project from a build stage, that project layer can be identical in hash if it's copied onto an updated system layer.
In practice, it's hard to get this to work because it's very easy in docker to accidentally trigger a rebuild for some random reason, but all the pieces are there.