Docker library snap example

This commit layers over the zlib PR (https://github.com/darinmorrison/docker-haskell/pull/7) and shows a potential example usage of the container using a Snap application.

This example *does not* use onbuild.

This example assumes the tag `haskell:7.8` is available.

It may be worth adding `ENV PATH /root/.cabal/bin:$PATH` or something similar to the base dockerfile. Another option would be to bias towards using `cabal build`.