Fix a bug in Docker file (#637)

Fix a bug.
Romove the echo command in line 36: RUN conda install pytorch-nightly cudatoolkit=${CUDA} -c pytorch 
To enable conda installation of pytorch-nightly.

docker/Dockerfile

@@ -33,7 +33,7 @@ RUN pip install ninja yacs cython matplotlib opencv-python
 
 # Install PyTorch 1.0 Nightly
 ARG CUDA
-RUN echo conda install pytorch-nightly cudatoolkit=${CUDA} -c pytorch \
+RUN conda install pytorch-nightly cudatoolkit=${CUDA} -c pytorch \
  && conda clean -ya
 
 # Install TorchVision master