Adding ffmpeg

Author: wcuk

Dockerfile

@@ -361,6 +361,7 @@ RUN pip install bcolz && \
     pip install allennlp && \
     # b/149359379 remove once allennlp 1.0 is released which won't cause a spacy downgrade.
     pip install spacy==2.2.3 && python -m spacy download en && python -m spacy download en_core_web_lg && \
+    apt-get install -y ffmpeg && \ 
     /tmp/clean-layer.sh
 
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