No-op changes to the repo produce large changes in the output manifests

[iaingalloway]

This commit is 1.3.0 (tagged): 16a9488
This commit is 1.3.1 (gitversion): 4558206

The second commit only changes the README.md

The build output should be identical

But the build output packages have a different manifest, and so a ~250MB layer in the final image that isn't shared.

I'd like to investigate why.