Funny this is here. Apptainer is Singularity, described here:

https://journals.plos.org/plosone/article?id=10.1371/journal...

If you ever use a shared cluster at a university or run by the government, Apptainer will be available, and Podman / Docker likely won't be.

In these environments, it is best not to use containers at all, and instead get to know your sysadmin and understand how he expects the cluster to be used.