add the option to use a `FORCE_CUDA` to force cuda installation on docker (#612)

* 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

INSTALL.md

@@ -58,13 +58,17 @@ unset INSTALL_DIR
 
 ### Option 2: Docker Image (Requires CUDA, Linux only)
 
-Build image with defaults (`CUDA=9.0`, `CUDNN=7`):
+Build image with defaults (`CUDA=9.0`, `CUDNN=7`, `FORCE_CUDA=1`):
 
     nvidia-docker build -t maskrcnn-benchmark docker/
     
 Build image with other CUDA and CUDNN versions:
 
-    nvidia-docker build -t maskrcnn-benchmark --build-arg CUDA=9.2 --build-arg CUDNN=7 docker/ 
+    nvidia-docker build -t maskrcnn-benchmark --build-arg CUDA=9.2 --build-arg CUDNN=7 docker/
+    
+Build image with FORCE_CUDA disabled:
+
+    nvidia-docker build -t maskrcnn-benchmark --build-arg FORCE_CUDA=0 docker/
     
 Build and run image with built-in jupyter notebook(note that the password is used to log in jupyter notebook):
 

demo/README.md

@@ -38,7 +38,8 @@ docker run --rm -it \
     -v /tmp/.X11-unix:/tmp/.X11-unix \
     --device=/dev/video0:/dev/video0 \
     --ipc=host maskrcnn-benchmark \
-    python demo/webcam.py --min-image-size 300
+    python demo/webcam.py --min-image-size 300 \
+    --config-file configs/caffe2/e2e_mask_rcnn_R_50_FPN_1x_caffe2.yaml
 ```
 
 **DISCLAIMER:** *This was tested for an Ubuntu 16.04 machine, 

docker/Dockerfile

@@ -47,6 +47,8 @@ RUN git clone https://github.com/cocodataset/cocoapi.git \
  && python setup.py build_ext install
 
 # install PyTorch Detection
+ARG FORCE_CUDA="1"
+ENV FORCE_CUDA=${FORCE_CUDA}
 RUN git clone https://github.com/facebookresearch/maskrcnn-benchmark.git \
  && cd maskrcnn-benchmark \
  && python setup.py build develop

setup.py

@@ -28,7 +28,7 @@ def get_extensions():
     extra_compile_args = {"cxx": []}
     define_macros = []
 
-    if torch.cuda.is_available() and CUDA_HOME is not None:
+    if (torch.cuda.is_available() and CUDA_HOME is not None) or os.getenv("FORCE_CUDA", "0") == "1":
         extension = CUDAExtension
         sources += source_cuda
         define_macros += [("WITH_CUDA", None)]