Docker support within the repository (#1732)

- Adds a Dockerfile within this repository
- GitHub actions to build and test the image

Dockerfile

@@ -0,0 +1,29 @@
+FROM ruby:2.7
+ENV DEBIAN_FRONTEND="noninteractive"
+
+RUN apt-get update && apt-get install -y \
+    libicu-dev \
+    cmake
+
+COPY Gemfile* /tmp/
+COPY gollum.gemspec* /tmp/
+WORKDIR /tmp
+RUN bundle install
+
+RUN gem install \
+    asciidoctor \
+    creole \
+    wikicloth \
+    org-ruby \
+    RedCloth \
+    bibtex-ruby \
+    && echo "gem-extra complete"
+
+WORKDIR /app
+COPY . /app
+RUN bundle exec rake install
+
+VOLUME /wiki
+WORKDIR /wiki
+COPY docker-run.sh /docker-run.sh
+ENTRYPOINT ["/docker-run.sh"]