Added Rackt example & tweaked Dockerfile

[leiDnedyA]

Response to #34.

Here's a demo of my branch running from a docker container:

To get React to import correctly, I had to tweak the Rackt module itself, as well as the Dockerfile. Here's all of the stuff that I did:

• Added <script> tag imports from the react and react-dom CDN links to the iFrame's index.html
• Forked and tweaked Rackt to use the global import from the script tag instead of an es6 import statement
• Set up the Dockerfile to clone my Rackt fork, checkout to the cdn-import branch, and install the package via raco pkg install
• Added a .dockerignore file (works exactly like .gitignore), so that build files will be generated via docker build, rather than being copied from the local build.