Remove the need for an external image registry for local development

At the moment, the requirements for running the operator seem to suggest that we need an external image registry for the setup to work. Although I agree with this setup for Production, we don't really need the registry for local development.

Like I did [here](https://github.com/etcd-io/etcd-operator/issues/130#issuecomment-3587025483), we should be able to remove the need for external registry via:
- `make docker-build` without `make docker-push`
- For `kind`, we can upload the image with `kind load docker-image`. I believe other local kubenetes setup should also have mechanism to do the same thing.
- Setting `imagePullPolicy: true` for the operator (config/manager/manager.yaml). I just raised a PR for this (see #246).

Let me know if this makes sense for you guys. I can also add a documentation for local development if you want.