Sql Alchemy compatibility

[frosk1]

I did not find any issue within this repository covering this, which makes me wonder how one could successfully run the docker images after build.

Airflow 1.10.9 does not support SQLAlchemy higher than 1.3.15! here is the corresponding issue: [/apache/airflow/issues/8211]

Airflow 1.10.10 does support higher SQLAlchemy version!

Within the master branch Dockerfile Airflow 1.10.9 is applied and SQLAlchemy> 1.3.15 this lead to non working webserver!

FIX as stated in the closed issue:

1. Keep Airflow 1.10.9 and put && pip install SQLAlchemy=='1.3.15' \ into the Dockerfile OR
2. Build Airflow 1.10.10 for compatibility with newer SQLAlchemy versions

[realyashnag]

This also addresses #535.