add dockerfile for local development

Dockerfile

+FROM ruby:2.6
+
+# throw errors if Gemfile has been modified since Gemfile.lock
+RUN bundle config --global frozen 1
+RUN gem install bundler:1.16.6
+
+WORKDIR /usr/src/app
+
+COPY Gemfile Gemfile.lock ./
+RUN bundle install
+
+COPY . .
+
+RUN bundle exec rake init
+
+EXPOSE 4000
+
+CMD ["bundle", "exec", "jekyll", "serve", "--host", "0.0.0.0"]
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