Build the image on the deployment server? Why not build somewhere else once and save time during deployments?

I'm most familiar with on-prem deployments and quickly realised that it's much faster to build once, push to registry (eg github) and docker compose pull during deployments.

I think the idea with unregistry is that you're still building somewhere else once but then instead of pushing everything to a registry once you push your unique layers directly to each server you're deploying.