Ability to define a tag name when deploying a prebuilt image from the Container Registry to Apps Platform

**What is the problem this feature would solve? Please describe.**

Deployments from DO Container Registry to Apps platform are limited and require manual input which is not ideal.
After contacting support i understand there are plans to support "deploy_on_push" for prebuilt images, but there's no ETA on that.

**Describe the solution you'd like**

Maybe it would be possible to include a new flag or some other way to specify the tag name when creating a new deployment using the cli.

Perhaps something like this
```
$ doctl apps create-deployment --tag={TAG_NAME}
```

which would update the app.spec
```
services:
image:
tag: {NEW_TAG_NAME}
```
before starting the deployment