Set maxsockets for npm install during build to avoid network errors

While trying to set up Shinobi on my new QNAP NAS I noticed images for arm64v8 are not being built since august 2023. I just reported this on issue #520 (closed), and since I was able to reproduce the issue locally I found out that setting --maxsockets 1 to npm prevents it.

Running with this change on Gitlab took a couple of retries but the pipeline passed.