CI deploy_demo: push core images to private registry

Closes #615 (closed).

The error states that it can't push to the private registry. That much I understand.

Why this has started failing recently? That I don't understand.

Limitations:

• docker login only supports one login at a time.
• Gitlab deploy tokens are not editable.

That being said, this solution is not perfect, but I welcome any more elegant suggestion. I've spent a sufficient 6 hours on this.

• Modify core docker-compose.yaml to allow pushing to a custom registry. Keep the current as a default value.
• Still push core images to the public registry so that they are available to the public.
• Push core and premium images to the private registry.
  • Because we can't push only the premium images, because it has services that depend_on core services.
  • Because, when deploying, we also can't pull using docker-compose from a single registry for the same reason.
  • Because we also can't push and pull to multiple registries because of the docker login limitation.
  • Because we also can't have the same deploy token on both registries.
• When deploying, pull from the private registry instead of the public registry, because we want premium too in the demo.