For testing the docker image creation process
If the Docker image is in the public registry then no token is required. Otherwise, one needs to generate a "read packages" token first and then set the Singularity environment variables as shown below in order to use the (private) image
export SINGULARITY_DOCKER_USERNAME=<username>
export SINGULARITY_DOCKER_PASSWORD=<read-packages token>
singularity run docker://ghcr.io/schmucr1/python-test-image:main python