Environment variables can leak into singularity container

[sverhoeven]

Environment variables set in Python process will get passed into the singularity container started by BmiClientSingularity.
This gives problems when env var value points to a path that is not available inside container.

Problem was detected with GDAL_DATA env var in eWaterCycle/ewatercycle#268 (comment) and was worked around in eWaterCycle/ewatercycle#268 (comment), by setting env var inside Dockerfile.

Possible solution could be to start container with --cleanenv arg, see https://singularity.hpcng.org/user-docs/3.8/cli/singularity_run.html?highlight=cleanenv#options