adding note about an edge scenario for outbound nsg

Author: Larry Franks

articles/machine-learning/how-to-enable-virtual-network.md

@@ -169,6 +169,33 @@ The NSG rule configuration in the Azure portal is shown in the following image:
 
 [![The outbound NSG rules for Machine Learning Compute](./media/how-to-enable-virtual-network/limited-outbound-nsg-exp.png)](./media/how-to-enable-virtual-network/limited-outbound-nsg-exp.png#lightbox)
 
+> [!NOTE]
+> If you plan on using default Docker images provided by Microsoft, and enabling user managed dependencies, you must also use a __Service Tag__ of __MicrosoftContainerRegistry.Region_Name__ (For example, MicrosoftContainerRegistry.EastUS).
+>
+> This configuration is needed when you have code similar to the following snippets as part of your training scripts:
+>
+> __RunConfig training__
+> ```python
+> # create a new runconfig object
+> run_config = RunConfiguration()
+> 
+> # configure Docker 
+> run_config.environment.docker.enabled = True
+> # For GPU, use DEFAULT_GPU_IMAGE
+> run_config.environment.docker.base_image = DEFAULT_CPU_IMAGE 
+> run_config.environment.python.user_managed_dependencies = True
+> ```
+>
+> Estimator training__
+> ```python
+> est = Estimator(source_directory='.', 
+>                 script_params=script_params, 
+>                 compute_target='local', 
+>                 entry_script='dummy_train.py', 
+>                 user_managed=True)
+> run = exp.submit(est)
+> ```
+
 ### User-defined routes for forced tunneling
 
 If you're using forced tunneling with the Machine Learning Compute, add [user-defined routes (UDRs)](https://docs.microsoft.com/azure/virtual-network/virtual-networks-udr-overview) to the subnet that contains the compute resource.