Add docker to preview instructions in the readme.

[andre-geldenhuis]

Getting the Jekyll preview stuff installed is a little bit of a pain. Personally I use a docker container to preview gh-pages builds. In the cloned workshop repo root I run:

docker run --rm --label=jekyll --volume=$(pwd):/srv/jekyll
-it -p 127.0.0.1:4000:4000 jekyll/jekyll

and installing docker is pretty simple - apparently even for windows, though I've not tried it.

If there was any interest in adding this to the readme, I'll happily prepare a pull request. If you think it adds too much cognitive overhead, that's fine too. I just find it much easier.

[gvwilson]

I think this is beyond most of our contributors, but if we ever add a lesson on Docker, we should revisit - thanks.