fix dockerfile

Dockerfile

 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 'npx forever --minUptime 7000 --spinSleepTime 2000 -o out.log -e err.log ./server/server.js' >> /usr/bin/start.sh
+RUN echo 'npx forever --minUptime 7000 --spinSleepTime 2000 -o out.log -e err.log /opt/app/server/server.js' >> /usr/bin/start.sh
 
 CMD ["/bin/sh","/usr/bin/start.sh"]