Fix docker registry in workflow

[ranisalt]

Pull Request Prelude

• [x] I have followed proper The Forgotten Server code styling.
• [x] I have read and understood the contribution guidelines before making this PR.
• [x] I am aware that this PR may be closed if the above-mentioned criteria are not fulfilled.

Changes Proposed

Issues addressed: the default registry, if unspecified, is Docker Hub. We want, however, to push them to the GitHub container registry at ghcr.io