az container create / azure container instances creation should automatically provide credentials for ACR images

**Is your feature request related to a problem? Please describe.**
`az container create` should automatically provide credentials for ACR images

**Describe the solution you'd like**
`az acr login` already obtains credentials for `docker` to log into ACR. Use these same credentials automatically for `az container create` if the image specified is from ACR.

**Describe alternatives you've considered**
Using the ACR admin password isn't recommended and using service principals is cumbersome. No good alternative exists.