Add dockerfile to folder structure

Author: PramodValavala-MSFT

articles/azure-functions/functions-reference-python.md

@@ -72,6 +72,7 @@ The recommended folder structure for a Python Functions project looks like the f
  | | - my_second_helper_function.py
  | - host.json
  | - requirements.txt
+ | - Dockerfile
  tests
 ```
 The main project folder (\_\_app\_\_) can contain the following files:
@@ -81,11 +82,13 @@ The main project folder (\_\_app\_\_) can contain the following files:
 * *host.json*: Contains global configuration options that affect all functions in a function app. This file does get published to Azure. Not all options are supported when running locally. To learn more, see [host.json](functions-host-json.md).
 * *.funcignore*: (Optional) declares files that shouldn't get published to Azure.
 * *.gitignore*: (Optional) declares files that are excluded from a git repo, such as local.settings.json.
+* *Dockerfile*: (Optional) used when publishing your project in a [custom container](functions-create-function-linux-custom-image.md).
 
 Each function has its own code file and binding configuration file (function.json). 
 
 When deploying your project to a function app in Azure, the entire contents of the main project (*\_\_app\_\_*) folder should be included in the package, but not the folder itself. We recommend that you maintain your tests in a folder separate from the project folder, in this example `tests`. This keeps you from deploying test code with your app. For more information, see [Unit Testing](#unit-testing).
 
+
 ## Import behavior
 
 You can import modules in your function code using both explicit relative and absolute references. Based on the folder structure shown above, the following imports work from within the function file *\_\_app\_\_\my\_first\_function\\_\_init\_\_.py*: