`run_debugger.sh`: add option to run builder container shell

`run_debugger.sh` is convenient to use for building irods, as it will build and run the core builder image and knows what arguments to pass to `docker`. It would be useful to extend this functionality to run a shell in the builder container, bypassing the image's entrypoint. This can come in handy for diagnosing build failures or building against unpublished externals packages, for example.