Add ffmpeg to docker image

Activating the media preview policy results in a dependency on ffmpeg in the next startup, and the backend erroring until ffmpeg is found. While this can be easily edited if it was enabled though a config file, if it was enabled through database configs, then the user could be stuck unless they found a docker image with ffmpeg or somehow modified the database configs. This PR adds ffmpeg to the dockerfile since the media preview proxy is a first party feature.