add instructions to run in linux

[bjrmatos]

based on this comment

[pofider]

You can check the jsreport official docker file which have this working.

These packages are required for electron on ubuntu and same on node debian based images
libgtk2.0-dev libxtst-dev libxss1 libgconf2-dev libnss3-dev libasound2-dev xvfb

Note that if you restart docker container, the xvfb fails to start. That is why there is trick with removing /tmp/.X99-lock file. This took me hours to find out, but now it seems to be working nice.

echo Trying to remove the lock on displat 99
rm /tmp/.X99-lock > /dev/null 2>&1

echo Running display 99
Xvfb :99 -screen 0 1024x768x24 > /dev/null 2>&1 &

echo Starting node.js
export DISPLAY=:99 && node "/usr/src/app/server.js"