Doesn't ADD do the download unconditionally but only invalidate the cache if the contents change?

The only good use for ADD that I've found is for invalidating git clones from GitHub:

    ADD https://api.github.com/repos/<user>/<repo>/git/refs/heads/<branch> version.json
    RUN git clone --depth=1 --single-branch --branch=<branch> https://github.com/<user>/<repo>.git