Docker image is missing gems

[Letme]

I am a Docker fan, so I was trying to run the project only from public image. While create passed, there are few other problems mostly about missing gems

$ docker run --rm -v $PWD:/Open-Publisher/Source -v $PWD/output:/Open-Publisher/Books -it ctarwater/open-publisher bundle exec rake pdf[all]
Could not find gem 'jekyll' in any of the gem sources listed in your Gemfile or available on this machine.
Run `bundle install` to install missing gems.

And when I tried to install the gems it failed as well (obviously ruby in docker container ruby 2.1.9p490 (2016-03-30 revision 54437) [x86_64-linux] is a bit old I assume)

$ docker run --rm -v $PWD:/Open-Publisher/Source -v $PWD/output:/Open-Publisher/Books -it ctarwater/open-publisher bundle install && rake pdf[all]
Fetching gem metadata from https://rubygems.org/...........
Fetching version metadata from https://rubygems.org/...
Fetching dependency metadata from https://rubygems.org/..
Resolving dependencies...
Installing rake 13.0.1

Gem::InstallError: rake requires Ruby version >= 2.2.
....
An error occurred while installing rake (13.0.1), and Bundler cannot continue.
Make sure that `gem install rake -v '13.0.1'` succeeds before bundling.

So maybe a solution is to just build a new container and re-upload it to DockerHub?

Thanks for the great project and idea, but I assume it needs some polishing from developer perspective...