[singularity] add documentation
Document singularity support from the admin perspective (configuration settings, etc)
Document singularity support from the user perspective (what changes are needed in my CWL, if anything, when my cluster uses singularity instead of docker?).
Document any limitations or differences in behavior when using singularity vs docker.
As part of the refactoring required for Singularity support, we have a slight behavior change in the loading of docker images from Keep (cf. https://dev.arvados.org/issues/17296#note-34):
- slight behavior change in loading docker images: before, the .tar file of the image had to be the first file in the collection, and that was loaded. Any subsequent .tar files were ignored. Now, it is an error if there is more than one .tar file. This change should be mentioned in the upgrade docs (though it's unlikely someone would run into it).