azurerm_linux_function_app with docker does not produce expected results

[elliotchaim]

Is there an existing issue for this?

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Terraform Version

1.1.7

AzureRM Provider Version

~>1.1.7

Affected Resource(s)/Data Source(s)

azurerm_linux_function_app

Terraform Configuration Files

resource "azurerm_linux_function_app" "example" {
  name                = "example"
  location            = azurerm_resource_group.example.location
  resource_group_name = azurerm_resource_group.example.name

  service_plan_id             = data.azurerm_service_plan.example.id
  storage_account_name        = azurerm_storage_account.example.name
  functions_extension_version = "~4"

  site_config {
    always_on = true
    ftps_state = "Disabled"

    application_stack {
      docker {
        registry_url      = "https://registry.hub.docker.com"
        image_name        = "username/imagename"
        image_tag         = "latest"
        registry_username = "username"
        registry_password = var.dockerhub_password
      }
    }
  }

  app_settings = {
    WEBSITE_WEBDEPLOY_USE_SCM           = true
    ASPNETCORE_ENVIRONMENT              = "Production"
    FUNCTIONS_WORKER_RUNTIME            = "dotnet-isolated"
    WEBSITES_ENABLE_APP_SERVICE_STORAGE = false
  }
}

### Debug Output/Panic Output

```shell
2022-04-21T05:56:19.959Z ERROR - Pull image threw Exception: Input string was not in a correct format.
2022-04-21T05:56:19.959Z INFO - Pulling image from Docker hub: https://registry.hub.docker.com/username/imagename:latest
2022-04-21T05:56:19.960Z ERROR - DockerApiException: Docker API responded with status code=BadRequest, response={"message":"invalid reference format"}

2022-04-21T05:56:19.961Z WARN - Image pull failed. Defaulting to local copy if present.
2022-04-21T05:56:19.963Z ERROR - Image pull failed: Verify docker image configuration and credentials (if using private repository)
2022-04-21T05:56:19.967Z INFO - Stopping site example because it failed during startup.
2022-04-21T05:58:46.398Z INFO - Pulling image: https://registry.hub.docker.com/username/imagename:latest
2022-04-21T05:58:46.400Z ERROR - DockerApiException: Docker API responded with status code=BadRequest, response={"message":"invalid reference format"}

### Expected Behaviour

1. The **Full Image Name and Tag** setting has a value of `username/imagename:latest`
2. The app service successfully pulls the docker image

### Actual Behaviour

1. The **Full Image Name and Tag** setting has a value of `https://username/imagename`
2. The app service is unable to pull the docker image

### Steps to Reproduce

1. `terraform apply`
2. Find the functions app in the Azure Portal
3. Click on **Deployment Center**
    a. Inspect the value of the **Full Image Name and Tag** setting
4. Click on **Logs** to view the app service output

### Important Factoids

_No response_

### References

NB: `azurerm_linux_web_app` resources configured with docker work as expected.

[elliotchaim]

Removing the https:// prefix from the registry_url fixed this issue, which is confusing for two reasons

1. It contradicts Microsoft's documentation
2. It is inconsistent with configuration of azurerm_linux_web_app resources

[github-actions]

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