There's a bug when converting APNG files to WEBP files, which
imagemagick delegates to ffmpeg. When doing 'nix build' and running the
result, or running pict-rs in the dev shell, it works fine. In the
container, this doesn't work at all. imagemagick complains that there's
no media to convert, implying ffmpeg has output a zero-sized file.
This work is helping to narrow down exactly what pict-rs needs to run,
though. This still needs to be tested against h264, h265, vp8, vp9 and
av1.
