docker file lacks ffmpeg

[Bovey0809]

Thanks for your error report and we appreciate it a lot.

Checklist

1. I have searched related issues but cannot get the expected help.
2. The bug has not been fixed in the latest version.

Describe the bug
The mmocr docker file can't passed "pytest" due to libgl.so.1 not found.

Reproduction

1. What command or script did you run?

docker build -t docker/ .
pytest

2. Did you make any modifications on the code or config? Did you understand what you have modified?
   No, I fixed it by adding "RUN apt-get install ffmpeg" to dockerfile.

Bug fix

RUN apt-get update && apt-get install -y git ninja-build libglib2.0-0 libsm6 libxrender-dev libxext6 ffmpeg\
    && apt-get clean \
    && rm -rf /var/lib/apt/lists/*

[innerlee]

Thanks for the proposal. (Yes the libgl.so.1 error is quite common for docker)