npm not found but installed from the shell script file in Dockerfile
The issue is that prereqs-ubuntu.sh uses bash to install the npm. While the RUN directive uses sh to run the commands. 

Where is the npm installed

root@2cd4a6af90f4:/app# type npm
npm is /root/.nvm/versions/node/v8.16.0/bin/npm


The PATH for the RUN directive

# echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin