How to debug Docker cache invalidation?
Solution 1:
Use binary search, with .dockerignore.
Add half your files to .dockerignore, and build the container. If it uses the cache for the COPY step, then you know the changed files are in the set you ignored, otherwise you know it's in the other half. Repeat this test with the set of files that has the change, until it's just one file/folder.
(Dear lazyweb: figure out some way to extend Docker to make this less painful!)