Bump to Alpine 3.12?

[p3palazzo]

Is there a reason why jekyll-docker is still on Alpine 3.11, other than "don't fix what's not broken"? I have been running my own fork of this repo on Alpine 3.12 and everything seems to work fine.

The reason I'm asking, in case anyone is interested, is that I built a custom image on top of jekyll-docker that uses the jekyll-pandoc plugin. The Pandoc docker image from which I'm copying the pandoc binary is built on Alpine 3.12 and requires the libffi version that is shipped with the up-to-date version of the OS.