Fix docker image to include pygments.rb

.gitignore

+/.idea
 /public
 _gen

Dockerfile

 
 FROM debian:stretch as hugo
 RUN apt-get -qq update \
-	&& DEBIAN_FRONTEND=noninteractive apt-get -qq install -y --no-install-recommends python-pygments git ca-certificates asciidoctor \
-	&& rm -rf /var/lib/apt/lists/*
+	&& DEBIAN_FRONTEND=noninteractive apt-get -qq install -y --no-install-recommends python-pygments git ca-certificates ruby \
+	&& rm -rf /var/lib/apt/lists/* \
+	&& gem install asciidoctor pygments.rb
 
 ENV HUGO_VERSION 0.54.0
 ENV HUGO_BINARY hugo_extended_${HUGO_VERSION}_Linux-64bit.deb

themes/book/layouts/partials/docs/git-footer.html

   <div>
     {{- $date := .AuthorDate.Local.Format (default "January 2, 2006" $.Site.Params.BookDateFormat) -}}
     <a href="{{ $.Site.Params.BookRepo }}/commit/{{ .Hash }}" title='Last modified {{ $date }} by {{ .AuthorName }}' target="_blank" rel="noopener">
-      <img src="{{ "svg/code-merge.svg" | relURL }}" /> Last modified at {{ $date }} in {{ .AbbreviatedHash }}
+      <img src="{{ "svg/code-merge.svg" | relURL }}" /> Last modified on {{ $date }} in {{ .AbbreviatedHash }}
     </a>
   </div>
   {{ end }}