Add Dockerfile for building yadm/jekyll image

This new image will be used for testing the website builds. In addition
to Jekyll functionality, the new image will have http-proofer and
yamllint.

Dockerfile

@@ -0,0 +1,18 @@
+FROM jekyll/jekyll:3.8.5
+MAINTAINER Tim Byrne <sultan@locehilios.com>
+
+# Convenience settings for the testbed's root account
+RUN echo 'set -o vi' >> /root/.bashrc
+
+# Create a flag to identify when running inside the yadm/jekyll image
+RUN touch /.yadmjekyll
+
+# Extra dependencies for testing
+RUN gem install html-proofer
+RUN apk add --update py-pip
+RUN pip install yamllint==1.15.0
+
+# # Gemfile dependencies (for speed)
+# COPY Gemfile /tmp/Gemfile
+# RUN bundle install --gemfile=/tmp/Gemfile
+# RUN rm -f /tmp/Gemfile*