Container Group: updating to use a Windows image

[tombuildsstuff]

The tests are currently failing for this since the Windows Docker container was pulled:

------- Stdout: -------
=== RUN   TestAccAzureRMContainerGroup_windowsBasic
--- FAIL: TestAccAzureRMContainerGroup_windowsBasic (58.02s)
    testing.go:459: Step 0 error: Error applying: 1 error(s) occurred:
        
        * azurerm_container_group.test: 1 error(s) occurred:
        
        * azurerm_container_group.test: containerinstance.ContainerGroupsClient#CreateOrUpdate: Failure responding to request: StatusCode=400 -- Original Error: autorest/azure: Service returned an error. Status=400 Code="InaccessibleImage" Message="The image 'winappimage:latest' in container group 'acctestcontainergroup-4456597369366035246' is not accessible. Please check the image and registry credential."
FAIL

with these changes the tests now pass:

$ acctests azurerm TestAccAzureRMContainerGroup_
=== RUN   TestAccAzureRMContainerGroup_linuxBasic
--- PASS: TestAccAzureRMContainerGroup_linuxBasic (65.69s)
=== RUN   TestAccAzureRMContainerGroup_linuxBasicUpdate
--- PASS: TestAccAzureRMContainerGroup_linuxBasicUpdate (67.84s)
=== RUN   TestAccAzureRMContainerGroup_linuxComplete
--- PASS: TestAccAzureRMContainerGroup_linuxComplete (70.88s)
=== RUN   TestAccAzureRMContainerGroup_windowsBasic
--- PASS: TestAccAzureRMContainerGroup_windowsBasic (70.94s)
=== RUN   TestAccAzureRMContainerGroup_windowsComplete
--- PASS: TestAccAzureRMContainerGroup_windowsComplete (59.38s)
PASS
ok  	github.com/terraform-providers/terraform-provider-azurerm/azurerm	334.768s

[mbfrahry]

LGTM

[ghost]

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