Docker build not invalidating cache when new files added

I have a Dockerfile that first copies over some common configuration files, then does more app specific things:

FROM postgres:9.4

COPY etc /etc
RUN apt-get update \
  && ...

From a clean slate, this works great. And if I modify files in the etc directory, docker picks up the change and invalidates the cache, rebuilding the image.

But if I’ve already cached this step before, and then I add new files to the etc directory (without modifying any existing ones), docker doesn’t detect that the cache needs to be invalidated:

Step 3 : COPY etc /etc
 ---> Using cache
 ---> 462e62492471

Which means my build will succeed, but the new files wont be present.

Is this a bug, or is there something about Docker image caching that I’m not understanding?

Note that the files I added were all directories (which themselves contain files), so perhaps the checksum that is computed for a COPY’d directory is not doing a recursive check.