WebApp:az webapp create should have a --docker-registry-server-url parameter

**Is your feature request related to a problem? Please describe.**

I want to deploy a multi-container web app but I ran into two issues.
1. `az webapp create` doesn't have a `--docker-registry-server-url` parameter but it does have a `--docker-registry-server-user` and a `--docker-registry-server-password`. This seems strange. What is the reasoning behind that parameter missing? And if there is one, what is the reasoning of having the other 2 parameters (user & password). I have some of the images from the docker-compose file in a private ACR.
2. `az webapp config container` set has all the 3 parameters mentioned above and I'm currently using that to set the `--docker-registry-server-url`.

But the strange thing is: If I create the webapp without providing my docker-compose file, some default image is pulled and started. After I set the correct configuration by using `az webapp config container set` I need to restart the app a couple of times to get the wanted images downloaded.

Considering I don't have the web app already created, I'm currently performing the following flow:
1. create the web app without docker registry credentials, but providing my docker-compose. This results in a bunch of errors until I provide the credentials
2. set up the credentials by using `az webapp config container set`

This flow results in a delayed start-up because it has to recover from the failures. If I wouldn't provide the docker-compose file I would instead get a default image pulled and that would start on its own and I would have to restart the web app resulting, again, in a delayed start-up time.

**Describe the solution you'd like**

Please consider adding a `--docker-registry-server-url` to the `az webapp create` command like you have in `az webapp config container set`.