docker-push-to-ecr: Add support for providing a custom ECR address

`auth-service` (and potentially others) will soon be deploying two Docker images to ECR (a "regular" image for us to use in production and an "on prem" image for customers). We could support this by using the `--suffix` flag (`--suffix=onprem`), but ideally we'd just have separate ECRs (`auth-service` and `auth-service-on-prem`). In order to support this here, we should create an `--ecr=` (or `--repo=`?) argument which will override the `{env}_ECR` environment variables.