npm install fails in jenkins pipeline in docker

I'm following a tutorial about Jenkins pipeline and I can get a "hello world" working under at node 6.10 docker container.

But, when I added a default EmberJS app (using ember init) to the repo and attempt to build that in the pipeline, it fails when running npm install (because of directory access issues). The Jenkinsfile can be seen here: http://ift.tt/2mxosJP

The error message printed by the build is (which is installed locally and run using java -jar jenkins.war on a Macbook, not relevant but included just in case) is:

npm ERR! Linux 4.9.12-moby
npm ERR! argv "/usr/local/bin/node" "/usr/local/bin/npm" "install"
npm ERR! node v6.10.0
npm ERR! npm  v3.10.10
npm ERR! path /.npm
npm ERR! code EACCES
npm ERR! errno -13
npm ERR! syscall mkdir

npm ERR! Error: EACCES: permission denied, mkdir '/.npm'
npm ERR!     at Error (native)
npm ERR!  { Error: EACCES: permission denied, mkdir '/.npm'
npm ERR!     at Error (native)
npm ERR!   errno: -13,
npm ERR!   code: 'EACCES',
npm ERR!   syscall: 'mkdir',
npm ERR!   path: '/.npm',
npm ERR!   parent: 'pipeline-tutorial' }
npm ERR! 
npm ERR! Please try running this command again as root/Administrator.