jenkins: fix npm build, removing docker -u

[aledsage]

npm writes to ~/.npm, and reads ~/.npmrc. But user 910 does
not exist in container, so has no home directory. Therefore
npm fails. Reverting -u for now.

e.g. https://builds.apache.org/view/B/view/Brooklyn/job/brooklyn-master-build-docker-pipeline/10/console failed with:

[INFO] --- frontend-maven-plugin:1.3:npm (npm install) @ brooklyn-ui-utils ---
[INFO] Running 'npm install' in /home/jenkins/jenkins-slave/workspace/brooklyn-master-build-docker-pipeline/brooklyn-ui/ui-modules/utils
[ERROR] Unhandled rejection Error: EACCES: permission denied, mkdir '/.npm'

This stackoverflow looks useful for fixing it longer term: https://stackoverflow.com/questions/46129443/sudo-permission-inside-docker-container-for-jenkins-pipeline.

We could npm config set cache /tmp and also export HOME=....

Or we could create a user with id 910:910 in the container, as part of the Dockerfile.

[kemitix]

Couldn't we mount a volume as /.npm and $HOME? I thought that was the intention behind docker having the -u option.