Fix _PIP_ADDITIONAL_REQUIREMENTS case for docker-compose
diff --git a/docs/apache-airflow/start/docker-compose.yaml b/docs/apache-airflow/start/docker-compose.yaml
index dadd1151513c8..aeaa724d3fdeb 100644
--- a/docs/apache-airflow/start/docker-compose.yaml
+++ b/docs/apache-airflow/start/docker-compose.yaml
@@ -235,6 +235,7 @@ services:
       _AIRFLOW_WWW_USER_CREATE: 'true'
       _AIRFLOW_WWW_USER_USERNAME: ${_AIRFLOW_WWW_USER_USERNAME:-airflow}
       _AIRFLOW_WWW_USER_PASSWORD: ${_AIRFLOW_WWW_USER_PASSWORD:-airflow}
+      _PIP_ADDITIONAL_REQUIREMENTS: ''
     user: "0:0"
     volumes:
       - .:/sources

Recent versions of Airflow do not allow to run `pip install` as
root but the `init` job runs as root so when the variable
_PIP_ADDITIONAL_REQUIREMENTS is set, the init container fails.

This PR forces _PIP_ADDITIONAL_REQUIREMENTS to be empty for the init
job.

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