Pipeline for Polymer Tests fails during bower install

[Pascal Laier]

I´m trying to create a pipline to run my Tests for a Polymer Web Component, but it fails when trying to run bower install with a Permission denied error:

bower moment#^2.18.1 install moment#2.18.1
bower prism#* install prism#1.8.1
bower EACCES EACCES: permission denied, mkdir '/opt/atlassian/pipelines/agent/iron-grid'
Stack trace:
/usr/lib/node_modules/bower/lib/node_modules/fstream/lib/dir-writer.js:35:25
/usr/lib/node_modules/bower/lib/node_modules/mkdirp/index.js:46:53
FSReqWrap.oncomplete (fs.js:123:15)
Console trace:
Error at StandardRenderer.error (/usr/lib/node_modules/bower/lib/renderers/StandardRenderer.js:81:37) 
at Logger.<anonymous> (/usr/lib/node_modules/bower/lib/bin/bower.js:110:26) at emitOne (events.js:96:13) 
at Logger.emit (events.js:188:7) at Logger.emit (/usr/lib/node_modules/bower/lib/node_modules/bower-logger/lib/Logger.js:29:39) 
at /usr/lib/node_modules/bower/lib/commands/index.js:48:20 at _rejected (/usr/lib/node_modules/bower/lib/node_modules/q/q.js:844:24) 
at /usr/lib/node_modules/bower/lib/node_modules/q/q.js:870:30 at Promise.when (/usr/lib/node_modules/bower/lib/node_modules/q/q.js:1122:31) 
at Promise.promise.promiseDispatch (/usr/lib/node_modules/bower/lib/node_modules/q/q.js:788:41)
System info:
Bower version: 1.8.0
Node version: 6.11.1
OS: Linux 4.12.10-coreos x64

My Pipeline configuration:

image: pascallaier/web-component-tester

pipelines:
  default:
    - step:
        caches:
          - bower
        script:
          - bower install --allow-root
          - xvfb-run polymer test

definitions:
  caches:
    bower: bower_components