change dockerfile to build using yarn

Signed-off-by: Alexander Schwartz <alexander.schwartz@gmx.net>

Dockerfile

@@ -32,6 +32,7 @@ RUN echo -n "Node.js version " && node --version \
   && hugo version
 
 RUN bundle \
+    && npm install -g yarn \
     && yarn \
     && hugo --gc --minify --destination public