Added target singularity-init and modified singularity-build to fix #22

[RubenRam]

Following up our conversation, here's my suggested change. Let me know if this suits what you had in mind for this modification.

[ksugar]

Thank you @RubenRam, I have made minor modifications to catch up with the latest dev branch of the main repository, in which apptainer commands are used instead of deprecated singularity commands.