Preview Environments not working for External Contributors

Seen in PR #109

```
ERROR: failed to solve: failed to push ghcr.io/gathering/tgno-frontend:preview-pr109: unexpected status from POST request to https://ghcr.io/v2/gathering/tgno-frontend/blobs/uploads/: 403 Forbidden
```

https://docs.github.com/en/actions/security-for-github-actions/security-guides/automatic-token-authentication#permissions-for-the-github_token