✅ Restore Apptainer tests

[shnizzedy]

Fixes

Related to #69 by @shnizzedy & Ammar_Ibrahim
Fixes #32 by @shnizzedy
Fixes #31 by @shnizzedy
Fixes #7 by @shnizzedy
Fixes #1 (comment) by @anibalsolon

Description

• Updates CI to build and test in Apptainer instead of Singularity
• Subclasses Apptainer backend from Singularity backend
• Creates mock C-PAC :latest and :nightly images that include C-PAC API but not the various fMRI dependencies (AFNI, ANTs, FreeSurfer, FSL, etc.)
• Fixes bugs in tests and in Singularity backend that were causing tests to fail

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the fix/FCP-INDI/cpac#69 branch of the repository.
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added tests for the changes I made (if applicable).
• I updated the changelog.
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no
  visible errors.

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