Exporting environment variable not working from shell script

Question

I have ubuntu image in docker, I'm trying to setup nodejs, phantomjs, npm etc..

and for PhantomJS I need to export QT_QPA_PLATFORM variable and set it's value to offscreen.

The problem is that when I run export QT_QPA_PLATFORM=offscreen using shell script, it doesn't work, here's my shell script:

apt-get update
apt-get install nodejs -y
apt-get install phantomjs -y
export QT_QPA_PLATFORM=offscreen

Any ideas?