Use GitHub Container Registry as default registry for main image

[LeoColomb]

This PR is switching the default registry for Netbox image from Docker Hub (docker.io) to GitHub Container Registry (ghcr.io).

The motivations are the following:

• GHCR is 100% free of use for egress, with almost nonexistent limits compared to Docker Hub. This helps to reduce rate limit errors of large deployments.
• Download speed is slightly better, especially for development in the GitHub context (mainly GitHub Codespaces and GitHub Actions)
• Container images management on GHCR is more transparent for other repositories as this one.
• The content is the same.

[RangerRick]

Love this! Let's do it.