singularity: expand comments, and reorder shell script

singularity/singularity.sh

 #!/bin/bash
 
-# use a dedicated "scratch disk"
-cd /tmp
-chmod 1777 /tmp 
+# clone the repo (with the singularity recipe file) to the scratch disk
+git clone https://jugit.fz-juelich.de/inm7/tools/htcondor-templates.git /tmp/htcondor-templates
+cd /tmp/htcondor-templates/singularity
 
-# download the singularity recipe file
-git clone https://jugit.fz-juelich.de/inm7/tools/htcondor-templates.git
-cd htcondor-templates/singularity
+# restore default permissions on the /tmp dir (from Condor's 0700). /tmp is
+# bind mounted inside of the singularity container, and will have permission
+# issues without this.
+chmod 1777 /tmp
 
 # build the singularity image file
+# (to gain this sudo access, it requires membership in the "singulars" group on
+# Juseless. If you need this, talk with Jan or Alex.)
 sudo singularity build sl.simg sl.def
 
 # copy the image file back to your directory