Feature request: --image-repository should not be always required when PackageType is Image

### Describe your idea/feature/enhancement

I came across an issue that when we use PackageType is Image, however we don't actually build/use image from docker file, it is pulling image straight from ECR. But somehow when doing deployment `sam deploy` it requires us to provide `--image-repository`

```yaml
GreetingFunctionUseDocker:
Type: AWS::Serverless::Function
Properties:
PackageType: Image
ImageUri: xxxxxxxxxx.dkr.ecr.us-west-2.amazonaws.com/greeting:latest
```

### Proposal

Only require ECR when building image from docker file.

Things to consider:
1. Will this require any updates to the [SAM Spec](https://github.com/awslabs/serverless-application-model)

### Additional Details