Add VOLUME to Dockerfile (#208)

Notes/exposes the intended volumes as per best practices.

See https://docs.docker.com/build/building/best-practices/#volume

Dockerfile

@@ -31,6 +31,8 @@ RUN mkdir -p /db /conf \
   && ln -s /conf /fredy/conf
 
 EXPOSE 9998
+VOLUME /db
+VOLUME /conf
 
 # Start application using PM2 runtime
 CMD ["pm2-runtime", "index.js"]