[AIRFLOW-5287] Base image for checklicence can be force pulled now

[potiuk]

Make sure you have checked all steps below.

Jira

• My PR addresses the following Airflow Jira issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
  • https://issues.apache.org/jira/browse/AIRFLOW-5287

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Code Quality

• Passes flake8

[potiuk]

A quick one - fixing re-pulling of latest debian-stretch and speeding up local rebuild of checklicence image.

[potiuk]

Ping here as well. This will speed up local pull and build a lot.