Npm cache fix

[hardillb]

• [x] Bugfix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)

Proposed changes

Set the global npm cache path to match the previous entry point.

Fix for https://github.com/node-red/node-red-docker/issues/326

Checklist

• [x] I have read the contribution guidelines
• [ ] For non-bugfix PRs, I have discussed this change on the mailing list/slack team.
• [ ] I have run grunt to verify the unit tests pass
• [ ] I have added suitable unit tests to cover the new/changed functionality

[knolleary]

Do we need to respin the images with this fix?

[hardillb]

Yes, but was wondering when 3.0.2 was going to ship (hopefully with the HTTP header parser fix)