This change adds support for building the eBook with Docker. Supporting Docker means
that users who want to build the eBook do not have to install Ruby/Bundler/Calibre in their
environments.
I'm a Docker n00b, so if there's a better way to handle file permissions on the output, I'm all
ears.
Build tested on Linux. Outputs were fine.
There is also a change to the pandoc outputs. It seems the `+smart` extension is not supported for epub/html[0]
in their latest versions.
[0] - https://pandoc.org/MANUAL.html#extension-smart
