Enhancing bootstrapping of external tools by adding Apptainer executable to the release process

[soronpo]

It's not a fully thought through yet, but I started getting interesting in leveraging containerized external executables through Apptainer (https://apptainer.org/). The great thing about Apptainer (vs. docker) is that it does not require a daemon, so everything can be run in usermode without installing anything on the computer that requires admin access. Only the apptainer executable is required and then many dependencies can be leveraged within mill.
Also thought about creating a wrapper library in Scala called scalapptainer :)
Would love to get some feedback if there is merit in the proposal.