Update Dockerfile. Use Jekyll official image

Dockerfile

@@ -1,23 +1,12 @@
-FROM ruby:2.1
+FROM jekyll/builder
-MAINTAINER mrafayaleem@gmail.com
-
-RUN apt-get clean \
-  && mv /var/lib/apt/lists /var/lib/apt/lists.broke \
-  && mkdir -p /var/lib/apt/lists/partial
-
-RUN apt-get update
-
-RUN apt-get install -y \
-    node \
-    python-pygments \
-  && apt-get clean \
-  && rm -rf /var/lib/apt/lists/
 
 WORKDIR /tmp
 ADD Gemfile /tmp/
 ADD Gemfile.lock /tmp/
 RUN bundle install
 
+FROM jekyll/jekyll
+
 VOLUME /src
 EXPOSE 4000