Use yarn

  * update Dockerfile
    - need the `apt-transport-https` until
    docker-library/buildpack-deps#55 is resolved
    - do not use `onbuild` and install yarn manually until
    nodejs/docker-node#243 is implemented
    - need `CMD` without `onbuild`
  * generate yarn.lock

Dockerfile

@@ -1,5 +1,27 @@
-FROM node:6-onbuild
+FROM node:6
+
+# TODO: remove once https://github.com/docker-library/buildpack-deps/issues/55 is resolved
+RUN apt-get update && apt-get install apt-transport-https
+
+# TODO: install yarn manually until https://github.com/nodejs/docker-node/issues/243 is implemented
+RUN curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | apt-key add -
+RUN echo "deb https://dl.yarnpkg.com/debian/ stable main" | tee /etc/apt/sources.list.d/yarn.list
+RUN apt-get update && apt-get install yarn
+
+RUN mkdir -p /usr/src/app
+WORKDIR /usr/src/app
+
+ONBUILD ARG NODE_ENV
+ONBUILD ENV NODE_ENV $NODE_ENV
+
+COPY package.json /usr/src/app/
+COPY yarn.lock /usr/src/app/
+
+RUN yarn install
+COPY . /usr/src/app
 
 RUN npm run build
 
 EXPOSE 8000
+
+CMD ["npm", "start"]