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

```pytb
Traceback (most recent call last):
File "/home/jenkins/sfvenv/bin/starforge", line 11, in
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
```

And in the case of Jenkins, this fails the entire build. Building sdists are of limited use anyway, since they are trivial to build manually and some packages do not create working sdists due to invalid manifests. I've temporarily disabled building sdists by commenting it out of `/home/jenkins/sfvenv/local/lib/python2.7/site-packages/starforge/commands/cmd_wheel.py`.