* add a FORCE_CUDA flag

Following discussion [here](https://github.com/facebookresearch/maskrcnn-benchmark/issues/167), this seemed the best solution

* Update Dockerfile

* Update setup.py

* add FORCE_CUDA as an ARG

* 	modified:   docker/Dockerfile
	modified:   setup.py

* small fix to readme of demo

* remove test print

* keep ARG_CUDA

* remove env value and use the one from ARG

* keep same formatting as source

* change proposed by @miguelvr

* Update INSTALL.md