text: Title: [BUG] Container usage from external registry 'docker.io' found cause a warning in the tests
  Body: ### Description
  <!--- Describe your issue/bug/request in detail -->
  We started to get a warning that can be seen [here](https://dev.azure.com/best-practices/recommenders/_build/results?buildId=60846&view=logs&j=475df697-7465-54db-fcd2-cb9bdea8ab03&t=b7db66b6-fa35-5c82-573e-eabcb50ded02)
  
  ```
  tools/docker/Dockerfile - Container usage from external registry 'docker.io' found.
  ##[warning]Container security analysis found 1 violations. This repo has one or more docker files having references to images from external registries. Please review https://aka.ms/containers-security-guidance to remove the reference of container images from external registries. Please reach out via teams (https://aka.ms/cssc-teams) or email (cssc@microsoft.com) for any questions or clarifications.
  
  ```
  This is weird because I don't see any reference to docker.io in the code
  
  ### In which platform does it happen?
  <!--- Describe the platform where the issue is happening (use a list if needed) -->
  <!--- For example: -->
  <!--- * Azure Data Science Virtual Machine. -->
  <!--- * Azure Databricks.  -->
  <!--- * Other platforms.  -->
  
  ### How do we replicate the issue?
  <!--- Please be specific as possible (use a list if needed). -->
  <!--- For example: -->
  <!--- * Create a conda environment for pyspark -->
  <!--- * Run unit test `test_sar_pyspark.py` with `pytest -m 'spark'` -->
  <!--- * ... -->
  
  ### Expected behavior (i.e. solution)
  <!--- For example:  -->
  <!--- * The tests for SAR PySpark should pass successfully. -->
  
  ### Other Comments
  