Bug: cannot run ubuntu-singularity as an instance

[ignatiusm]

ubuntu-singularity/README.md includes instructions for running a singularity container as an instance.

Following these instructions leads to no output to log files, no long running processes in the instance, and no evidence of runner being registered with GitHub.

I did some digging into this and noticed that the SingularityCE docs mention that a %startscript section is needed in the .def file to specify what is run when theinstance start command is issued (see documentation).

Fix: add a %startscript section to the .def file.