[ECR] [request]: support custom domains, or alternate URIs for repositories

**Tell us about your request**
Currently a repository URI looks like this: `.dkr.ecr..amazonaws.com/`. Account ID and region might be movable parts which has negative effects for the following scenarios described. It would be helpful to be able to define an alternate URI for ECR repositories.

**Which service(s) is this request for?**
ECR, (.. and maybe other container services?)

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
Our team provides a docker image for ~12 other teams that acts as a build tool for frontend resources within their pipelines. We identified different disaster recovery scenarios where the current ECR URI is a disadvantage:

(1) Unavailability of ECR within the specified region
- For this case *all* teams would have to change the URI of our repository or won`t be able to deploy frontend resources instead, until ECR is reachable again.

(2) Disaster recovery for the ECR account
- When the account which contains the ECR repository is (e.g.) compromised or we have to do a complete disaster recovery for other reasons, then *all* teams would have to change the URI of our repository. This is a dependency we could prevent..

An alternate repository URI could be a fixed interface for other consumers. Changes for account ID or region behind this part would not affect them anymore.

**Are you currently working around this issue?**
One way we had seen to "solve" this, is to use an nginx as reverse proxy for the ECR, but this is an effort we don`t want to practice.

**Additional context**
This [topic](https://forums.aws.amazon.com/thread.jspa?start=0&threadID=223934&tstart=0) from January 2016 also describes some pain points with this.

**Attachments**
- none

Start by reviewing the ECR URI format and the disaster-recovery scenarios described in the issue, including the referenced AWS forum topic and the nginx reverse-proxy workaround. Done means defining a supported alternate repository URI whose consumers are unaffected by changes to the ECR account or region.

Written by the indexing model from the issue text.