run_debugger.sh: add option to run builder container shell

[SwooshyCueb]

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 This can come in handy for diagnosing build failures or building against unpublished externals packages, for example.