Fix update-demo to work correctly with a web browser.

The built in go http.Fileserver handles If-Modified-Since in the
header by comparing it with the last modified date on the
file/directory. Since the file is unaltered when we switch to a
new container, that results in 304s when trying to fetch data.json, so
the browser doesn't fetch the new picture after doing a rolling update.

Separately, the Dockerfile ADD command gets misinterpreted on older
clients as `copy html/data.json into a directory called 'html/kitten.jpg '`,
instead of the `copy these 2 files into root`. Changed to separate
commands using COPY, per [dockerfile best practices](https://docs.docker.com/articles/dockerfile_best-practices/#add-or-copy).