Document XDG_RUNTIME_DIR related troubleshooting

[dtrudg]

See: https://github.com/sylabs/singularity/issues/5068

It's possible for an incorrect XDG_RUNTIME_DIR for the current user (happens with su and sudo) can lead to permission issues on directories that containers/image wants to access.

Write up some troubleshooting info about this, setting XDG_RUNTIME_DIR explicitly in flows where it may become incorrect / unset.