Avoid manually mirroring Elsa image

[DoxasticFox]

This is the part of the readme I don't like. That should happen automatically.

[andrewpatto]

Are there any supported container registries other than gcr.io?

(Because I'm happy to get the elsa build process to push anywhere that is suitable)

[DoxasticFox]

Ghcr might be supported by kubernetes. Mirroring was needed when I tried an initial deployment on Cloud Run. I didn't think to try ghcr again when I moved to kubernetes, so I stuck with gcr. It's possible this ticket can be solved simply by changing the image URL and deleting the part of the readme advocating mirroring.