🐛 Fix cpac --platform singularity run without instance not found error

[shnizzedy]

Fixes

Fixes #69 by @shnizzedy & Ammar_Ibrahim

Description

• When running a C-PAC Apptainer/Singularity container through the Python API, give the path to the image (which works in old and new versions of the platforms) instead of a running-instance object (which only works in newer versions).
• Update README.

Tests

• ✅ Restore Apptainer tests #70

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the develop branch of the repository.
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• 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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