Really? What do your dockerfiles look like? Most of the ones I've seen in the wild look something like:
FROM debian:jessie
RUN apt-get install -y somepkg
...
The answer is that `docker build` will produce a different image. In fact, very few docker image builds I've seen are idempotent. They're not declarative, they're not reproducible, merely the output (the docker image itself) is itself able to be run reproducibly. The actual image definition, not so much.
Creating idempotent shell scripts is no harder than creating an idempotent dockerfile. Both are the same problem. A dockerfile is almost entirely the same as a shell script; it copies files around, it runs commands in an environment, and that's all.