Update 'Dockerfile'

Dockerfile

@@ -2,10 +2,14 @@ FROM alpine:3.4
 
 MAINTAINER Hanjo Meinhardt <hanjo@bunix.de>
 
+ENV dev_packages="ca-certificates build-base libffi-dev ruby-dev ruby-rdoc ruby-irb"
+  
 RUN apk update && apk upgrade
   
-RUN apk install ruby  
+RUN apk add ruby ruby-json ${dev_packages}
   
 RUN rm -rf /var/cache/apk/*
 
 RUN gem install jekyll
+  
+RUN apk del ${dev_packages}