Do not install on run (#30)

* Do not install on run

The docker image should contains already resolved dependencies.

No need for a start.sh, you could just run node directly.

* Simplify Dockerfile

Dockerfile

 FROM node:10-alpine
 
-WORKDIR /opt/app
 COPY . /opt/app
+WORKDIR /opt/app
+RUN npm install --production
 
 ENV PORT=80
 EXPOSE 80
 
 VOLUME /opt/app/server-data
 
-RUN touch /usr/bin/start.sh # this is the script which will run on start
-
-RUN echo 'npm install --production' >> /usr/bin/start.sh
-RUN echo 'node /opt/app/server/server.js' >> /usr/bin/start.sh
-
-CMD ["/bin/sh","/usr/bin/start.sh"]
+CMD ["npm", "start"]