add docker-serve make target for #238
add docker-serve make target for #238

If Docker is installed, `make docker-serve` will download and start a
Jekyll container to build and serve the site with hot reloading.

JEKYLL_VERSION should be synced with https://pages.github.com/versions/

Reproducible-Science-Curriculum/rr-automation,Reproducible-Science-Curriculum/rr-literate-programming,Reproducible-Science-Curriculum/rr-literate-programming,Reproducible-Science-Curriculum/rr-automation