[Enhancement] Add mmcv arg expansion (#748)

* Add dynamic mmcv install

* Declare ARG after FROM

docker/serve/Dockerfile

@@ -24,7 +24,9 @@ RUN export FORCE_CUDA=1
 RUN pip install torchserve torch-model-archiver
 
 # MMLAB
-RUN pip install mmcv-full==${MMCV} -f https://download.openmmlab.com/mmcv/dist/cu101/torch1.6.0/index.html
+ARG PYTORCH
+ARG CUDA
+RUN ["/bin/bash", "-c", "pip install mmcv-full==${MMCV} -f https://download.openmmlab.com/mmcv/dist/cu${CUDA//./}/torch${PYTORCH}/index.html"]
 RUN pip install mmsegmentation==${MMSEG}
 
 RUN useradd -m model-server \