check on the client side that there is a Dockerfile

 so we don't upload a huge stack of files, only to realise we can't do anything

Closes #2319

I think that i count 3 places in the code that have the literal string ``Dockerfile`` - I wonder if thats getting to a DRYlimit.

at the moment, i feel that if you put a Dockerfile into ``/``, then you are expecting docker to tar up your complete fs - I'm sure as I do more ``dind``, i might change my opinion :)