Adds Dockerfile for site testing and development

Adds a dockerfile so the site can easily be tested (and deployed as well, if desired). 
Includes all necessary prerequisites, builds the site using jekyll from the git source, and by default runs the site under apache.

To build container and run in apache (on port 80):
`docker build -t local/www.htslib.org . && docker run -p 8000:80 -t -i local/www.htslib.org`

To build container and run using `jekyll serve` (on port 4000):
`docker build -t local/www.htslib.org . && docker run -p 8000:4000 -t -i local/www.htslib.org jekyll serve`

To build container and drop into a shell:
`docker build -t local/www.htslib.org . && docker run -p 8000:4000 -t -i local/www.htslib.org /bin/bash`