Clean PYTHONPATH and setup GWMS environment in Singularity
PYTHONPATH could interfere w/ the image, further, it could contain references to libraries that are OS specific, so it should be unset before starting the image.
GWMS provides binaries and utilities that could be useful for the job also in a singularity image (e.g. "pychirp").
The glidein should prepare a directory, mounted in a standard path with all (binaries, libraries, python libraries, .... This will be mounted by default but there would be the possibility to disable it (no mounting, no environment).