[ENH] add Apptainer definition

[Remi-Gau]

• closes #1240

[pep8speaks]

Hello @Remi-Gau! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file docs/source/conf.py:

Line 56:80: E501 line too long (84 > 79 characters)