fixed Dockerfile to run demo script in docker container (open-mmlab#1568)

* fixed Dockerfile to run demo script

* changed from libgl1-mesa-dev to libgl1-mesa-glx

Author: atinfinity

docker/Dockerfile

@@ -12,7 +12,7 @@ ENV CMAKE_PREFIX_PATH="$(dirname $(which conda))/../"
 RUN apt-key adv --fetch-keys https://developer.download.nvidia.com/compute/cuda/repos/ubuntu1804/x86_64/3bf863cc.pub
 RUN apt-key adv --fetch-keys https://developer.download.nvidia.com/compute/machine-learning/repos/ubuntu1804/x86_64/7fa2af80.pub
 
-RUN apt-get update && apt-get install -y git ninja-build libglib2.0-0 libsm6 libxrender-dev libxext6 \
+RUN apt-get update && apt-get install -y git ninja-build libglib2.0-0 libsm6 libxrender-dev libxext6 libgl1-mesa-glx \
     && apt-get clean \
     && rm -rf /var/lib/apt/lists/*