oci authenticator: Accept the right docker manifest when authenticating

Without this I got for the fedora registry:

```
getting token for https://registry.fedoraproject.org/v2/f32/flatpak-runtime/manifests/sha256:bd83b4f6974094848efac22b933419c1dbe11b553def148a82f821faf595de8a
F: Anonymous authentication failed: Unexpected response status 404 from repo
```
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f