Add app user

Author: gdiggs

Dockerfile

@@ -8,6 +8,9 @@ RUN apk --update add ruby ruby-dev ruby-bundler build-base && \
     bundle install -j 4 && \
     apk del build-base && rm -fr /usr/share/ri
 
+RUN adduser -u 9000 -D app
+USER app
+
 COPY . /usr/src/app
 
 CMD ["/usr/src/app/bin/codeclimate-rubocop"]