Adds missing variable for force pull base image variable (#14901)
Adds missing variable for force pull base image variable (#14901)

The variable's default value was not set, thus failing pre-commit
scripts in case image was not built recently.
bolkedebruin/airflow,danielvdende/incubator-airflow,lyft/incubator-airflow,nathanielvarona/airflow,danielvdende/incubator-airflow,apache/airflow,Acehaidrey/incubator-airflow,apache/incubator-airflow,sekikn/incubator-airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,dhuang/incubator-airflow,apache/airflow,apache/airflow,apache/airflow,nathanielvarona/airflow,dhuang/incubator-airflow,lyft/incubator-airflow,nathanielvarona/airflow,lyft/incubator-airflow,mistercrunch/airflow,Acehaidrey/incubator-airflow,bolkedebruin/airflow,Acehaidrey/incubator-airflow,nathanielvarona/airflow,cfei18/incubator-airflow,apache/airflow,dhuang/incubator-airflow,sekikn/incubator-airflow,bolkedebruin/airflow,mistercrunch/airflow,nathanielvarona/airflow,danielvdende/incubator-airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,Acehaidrey/incubator-airflow,dhuang/incubator-airflow,mistercrunch/airflow,danielvdende/incubator-airflow,Acehaidrey/incubator-airflow,sekikn/incubator-airflow,apache/incubator-airflow,bolkedebruin/airflow,apache/airflow,danielvdende/incubator-airflow,lyft/incubator-airflow,bolkedebruin/airflow,apache/incubator-airflow,cfei18/incubator-airflow,sekikn/incubator-airflow,cfei18/incubator-airflow,nathanielvarona/airflow,cfei18/incubator-airflow,apache/incubator-airflow,mistercrunch/airflow