Optimize Dockerfile (#1306)

docker/Dockerfile

@@ -4,10 +4,12 @@ ARG CUDNN="7.5"
 
 FROM pytorch/pytorch:${PYTORCH}-cuda${CUDA}-cudnn${CUDNN}-devel
 
-RUN apt-get update && apt-get install -y libglib2.0-0 libsm6 libxrender-dev libxext6
+RUN apt-get update && apt-get install -y libglib2.0-0 libsm6 libxrender-dev libxext6 \
+ && apt-get clean \
+ && rm -rf /var/lib/apt/lists/*
 
 # Install mmdetection
-RUN conda install cython -y
+RUN conda install cython -y && conda clean --all
 RUN git clone https://github.com/open-mmlab/mmdetection.git /mmdetection
 WORKDIR /mmdetection
-RUN pip install -e .
+RUN pip install --no-cache-dir -e .