Accept ProRes files (need -pix_fmt). Fix scale expression.

[stuartnlevy]

ffmpeg could fail to create a "baseline"-profile MP4 if the source movie uses a pixel format other than 4:2:0 (many but not all formats use 4:2:0).
ffmpeg's error report is buried in lots of other output, but it looks like:
x264 [error]: baseline profile doesn't support 4:2:2

For example, movies in one of the Apple ProRes formats have 4:2:2 or 4:4:4 pixel formats. Though these might be acceptable in non-"baseline" MP4 profiles, the ffmpeg H.264 wiki page ( https://trac.ffmpeg.org/wiki/Encode/H.264 ) warns that some players won't be able to play them. It suggests the "-pix_fmt yuv420p" ffmpeg option to convert to the baseline pixel format.

Also, in trying to run the ffmpeg command in the Dockerfile, we get errors related to parsing the option
-scale -1:'min(ih,360)'
This mysteriously seems to work if the above is changed to
-scale -1:'min(ih,360)'

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