Command to configure web app to pull image:tag not working as expected (in how to use system MI doc)

The following refers to the doc: https://github.com/Azure/app-service-linux-docs/blob/master/HowTo/use_system-assigned_managed_identities.md.

After running through the setup for using system managed identities with a linux web app, I noticed that the step that configures the web app to pull the image:tag was not working for me. After running that command with my image and tag specified, the portal always showed "myapp" and "latest" as the repo and tag. The same is reflected in the logs.

More specifically, the following line does not seem to work as intended:

```bash
#Configure the ACR, Image and Tag to pull
az resource update --ids $Webapp_Config --set properties.linuxFxVersion=$FX_Version -o none --force-string
```

Example error I am seeing in logs:

I am able to get the flow working if instead use the following command. The Azure CLI does still output a warning message saying that admin needs to be enabled for ACR. However, I have not enabled admin and it works despite the warning.

```bash

# configure the web app to pull the above image name + tag from ACR
# you can ignore the error that says to enable admin on acr
az webapp config container set -n $Web_Name -g $RG_Name \
-i ${ACR_URL}/${Image} \
-r $ACR_URL

```