Can't build sdists if the imageset doesn't contain a docker image #157

Traceback (most recent call last):
  File "/home/jenkins/sfvenv/bin/starforge", line 11, in <module>
    sys.exit(starforge())
  File "/home/jenkins/sfvenv/local/lib/python2.7/site-packages/click/core.py", line 722, in __call__
    return self.main(*args, **kwargs)
  File "/home/jenkins/sfvenv/local/lib/python2.7/site-packages/click/core.py", line 697, in main
    rv = self.invoke(ctx)
  File "/home/jenkins/sfvenv/local/lib/python2.7/site-packages/click/core.py", line 1066, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/jenkins/sfvenv/local/lib/python2.7/site-packages/click/core.py", line 895, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/jenkins/sfvenv/local/lib/python2.7/site-packages/click/core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "/home/jenkins/sfvenv/local/lib/python2.7/site-packages/click/decorators.py", line 64, in new_func
    return ctx.invoke(f, obj, *args[1:], **kwargs)
  File "/home/jenkins/sfvenv/local/lib/python2.7/site-packages/click/core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "/home/jenkins/sfvenv/local/lib/python2.7/site-packages/starforge/commands/cmd_wheel.py", line 108, in cli
    itervalues(wheel_config.images))[0]
IndexError: list index out of range