[ENH] initial docker / apptainer build files

[bcmcpher]

This is a first draft of a Dockerfile / apptainer .def file for creating a nipoppy container bundled with apptainer.

I just wanted to track it until a need for such a thing arises.

• PR has an interpretable title with a prefix (e.g. [BUG], [DOC], [ENH], [MAINT])\
• PR links to GitHub issue with mention Closes #XXXX
• Tests pass
• Checks pass

For new features:

• Tests have been added