Make the container image registry configurable

[samos123]

Right now the container image registry is not configurable. This prevents it from being used in environments like AWS and also prevents companies that want to use their own registry from using substratus.

There should be a configuration option in the controller itself to set the image registry.

[nstogner]

The registry url is configurable using this env variable on the controller: https://github.com/substratusai/substratus/blob/20b755fdc1d85fb9b800a7eb13610a027eeb3545/internal/cloud/common.go#L15

However, we have never tried using any registry other than Google Artifact Registry.