Defining PROD_LIBS in same ENV block as PYTHONPATH results in missing `requirements.txt` entries.

[metasim]

The driver/Dockerfile template has the following definition:

ENV \
    PROD_LIBS=/build/prod \
    PYTHONPATH=$PYTHONPATH:/$PROD_LIBS/lib/python3.6/site-packages:/home/cognition-datasources/spatial-db/lambda_db \
    LAMBDA_DB_PATH=/home/cognition-datasources/spatial-db/lambda_db/database.fs

Due to the way that Docker evaluates environment variable definitions, PROD_LIBS is not in scope when PYTHONPATH is set, and therefore the site-packages under /build/prod doesn't end up in the PYTHONPATH. This results in dependencies needed by custom drivers not being available in the run time.

You can verify this outcome by running the following:

$ docker run --rm -v $PWD:/home/cognition-datasources -it earthcache-driver:latest bash
bash-4.2# echo $PYTHONPATH

result:

/var/runtime://lib/python3.6/site-packages:/home/cognition-datasources/spatial-db/lambda_db

The fix is to break out the definition of PROD_LIBS into a separate layer:

ENV PROD_LIBS=/build/prod
ENV \
    PYTHONPATH=$PYTHONPATH:/$PROD_LIBS/lib/python3.6/site-packages:/home/cognition-datasources/spatial-db/lambda_db \
    LAMBDA_DB_PATH=/home/cognition-datasources/spatial-db/lambda_db/database.fs