fix bug in Dockerfile

facebookresearch#584
facebookresearch#617

Author: raphaelw

backend.Dockerfile

@@ -32,7 +32,9 @@ RUN pip install --upgrade pip setuptools
 RUN pip install -e ".[interactive-demo]"
 
 # https://github.com/Kosinkadink/ComfyUI-VideoHelperSuite/issues/69#issuecomment-1826764707
-RUN rm /opt/conda/bin/ffmpeg && ln -s /bin/ffmpeg /opt/conda/bin/ffmpeg
+# RUN rm /opt/conda/bin/ffmpeg && ln -s /bin/ffmpeg /opt/conda/bin/ffmpeg
+# https://github.com/facebookresearch/sam2/issues/584
+RUN [ -f /opt/conda/bin/ffmpeg ] && rm /opt/conda/bin/ffmpeg; ln -s /bin/ffmpeg /opt/conda/bin/ffmpeg
 
 # Make app directory. This directory will host all files required for the
 # backend and SAM 2 inference files.