Optimize context sent for docker build (#17415)
Optimize context sent for docker build (#17415)

The `provider_packages` folder is not needed during docker build
and it often contains copied sources when building the provider
packages. Also _doctree folder is prepared during documentation
building. This makes `docker build` wait for a few seconds when
those directories have a lot of files.

Excluding those decreases docker build overhead significantly.
danielvdende/incubator-airflow,nathanielvarona/airflow,nathanielvarona/airflow,apache/incubator-airflow,bolkedebruin/airflow,lyft/incubator-airflow,dhuang/incubator-airflow,apache/airflow,nathanielvarona/airflow,cfei18/incubator-airflow,bolkedebruin/airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,Acehaidrey/incubator-airflow,dhuang/incubator-airflow,bolkedebruin/airflow,danielvdende/incubator-airflow,lyft/incubator-airflow,nathanielvarona/airflow,lyft/incubator-airflow,Acehaidrey/incubator-airflow,apache/incubator-airflow,mistercrunch/airflow,dhuang/incubator-airflow,Acehaidrey/incubator-airflow,apache/airflow,nathanielvarona/airflow,apache/airflow,bolkedebruin/airflow,danielvdende/incubator-airflow,mistercrunch/airflow,danielvdende/incubator-airflow,bolkedebruin/airflow,apache/airflow,apache/airflow,danielvdende/incubator-airflow,apache/airflow,cfei18/incubator-airflow,apache/incubator-airflow,Acehaidrey/incubator-airflow,mistercrunch/airflow,nathanielvarona/airflow,lyft/incubator-airflow,mistercrunch/airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,apache/incubator-airflow,dhuang/incubator-airflow,cfei18/incubator-airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow