Update Dockerfile

Dockerfile

@@ -8,7 +8,7 @@ WORKDIR /app
 COPY . /app
 
 # 安装依赖
-RUN add-apt-repository ppa:kirillshkrogalev/ffmpeg-next & apt-get update & apt-get install ffmpeg
+RUN echo "deb http://archive.debian.org/debian jessie-backports main" | tee /etc/apt/sources.list.d/jessie-backports.list & apt-get update & apt-get install ffmpeg
 RUN pip install --no-cache-dir gunicorn
 RUN pip install --no-cache-dir -r requirements.txt
 RUN pip install --no-cache-dir flask