Installation workflow for easyconfigs using the Apptainer easyblock

[bedroge]

We need an installation workflow for easyconfigs that make use of the new Apptainer easyblock. This is not entirely straightforward, since our current build_containter.sh script already does the installations in a container, and the easyblock would then try to run Apptainer inside Apptainer. This may even work (see https://apptainer.org/docs/admin/1.1/installation.html#installation-on-linux), but it's currently not installed in our container, so this will have to be done and tested (especially if it works well with fuse-overlays).

Alternatively, we would need some other mechanism. Some possibilities:

• make a script that would be run on the Stratum 0, and which opens a transaction, does the installation, and publishes the transaction (would also prevent us from possibly having to download large containers, extract them, tar them again, ship them to the stratum0, extract them again)
• if nested containers cause issues for fuse-overlayfs, we could perhaps just bind mount an (empty) host directory as /cvmfs/hpc.rug.nl/containers, and use that for the actual installation. These installations should be stand-alone ones anyway (i.e. no deps), so this should probably work.

[bedroge]

Gave it a quick try by installing apptainer in our build container, and that does seem to work nicely. So perhaps that's the easiest solution anyway, just have to make the build script aware that it should look for these installations in another directory, and the ingestion script will have to modified as well.