Auto-init Apptainer and use it for tool dependency resolution

[natefoo]

The current default config initializes (installs) and uses Conda for dependencies. I believe we are at the point with our container build and distribution architecture that we should be preferring containers instead. This should generally be faster, more reliable, and more reproducible, as well. Apptainer has made great strides in unprivileged, unpackaged, relocatable, non-setuid installation, so it would be trivial to get a working Apptainer binary on a fresh install.

For this to work nicely, the cached_mulled_singularity resolver ought to be able to fetch from a URL, i.e. https://depot.galaxyproject.org/singularity. As things exist right now your options are to mount CVMFS or convert images from docker (with or without caching).

I guess the caveat is that this only applies to Linux - on macOS or WSL we probably want different defaults.

[natefoo]

Because the install-unprivileged.sh script requires rpm2cpio which is not likely to be installed on Debian/Ubuntu, I created a repo to auto-build these: https://github.com/galaxyproject/apptainer-build-unprivileged/. Galaxy should be able to fetch directly from the latest release (or we can pin it to a version). If that works well we should probably add a cron workflow to automatically build new releases.