Add ffmpeg to dockerfile/image in order to allow video preview thumbnails

[GUNT0815]

If you want to see previews for vid files when you have set

'enable_previews' => true,
'enabledPreviewProviders' =>
array (
0 => 'OC\Preview\TXT',
1 => 'OC\Preview\MarkDown',
2 => 'OC\Preview\OpenDocument',
3 => 'OC\Preview\PDF',
4 => 'OC\Preview\MSOffice2003',
5 => 'OC\Preview\MSOfficeDoc',
6 => 'OC\Preview\PDF',
7 => 'OC\Preview\Image',
8 => 'OC\Preview\Photoshop',
9 => 'OC\Preview\TIFF',
10 => 'OC\Preview\SVG',
11 => 'OC\Preview\Font',
12 => 'OC\Preview\MP3',
13 => 'OC\Preview\Movie',
14 => 'OC\Preview\MKV',
15 => 'OC\Preview\MP4',
16 => 'OC\Preview\AVI',
),

in the config file, you need ffmpeg to be installed in the docker image.