I installed opencv:x64-linux. During the installation, OpenCV downloads opencv_ffmpeg.dll, which is probably for Windows.
I didn't get any build errors afterwards, but ffmpeg doesn't appear to be working.

Starting package 5/5: opencv:x64-linux
Building package opencv[core,ffmpeg,jpeg,png,tiff]:x64-linux...
[...]
-- Downloading https://raw.githubusercontent.com/opencv/opencv_3rdparty/0a0e88972a7ea97708378d0488a65f83e7cc5e69/ffmpeg/opencv_ffmpeg.dll...
-- Downloading https://raw.githubusercontent.com/opencv/opencv_3rdparty/0a0e88972a7ea97708378d0488a65f83e7cc5e69/ffmpeg/opencv_ffmpeg.dll... OK
-- Testing integrity of downloaded file...
-- Testing integrity of downloaded file... OK
-- Downloading https://raw.githubusercontent.com/opencv/opencv_3rdparty/0a0e88972a7ea97708378d0488a65f83e7cc5e69/ffmpeg/opencv_ffmpeg_64.dll...
-- Downloading https://raw.githubusercontent.com/opencv/opencv_3rdparty/0a0e88972a7ea97708378d0488a65f83e7cc5e69/ffmpeg/opencv_ffmpeg_64.dll... OK
-- Testing integrity of downloaded file...
-- Testing integrity of downloaded file... OK
-- Downloading https://raw.githubusercontent.com/opencv/opencv_3rdparty/0a0e88972a7ea97708378d0488a65f83e7cc5e69/ffmpeg/ffmpeg_version.cmake...
-- Downloading https://raw.githubusercontent.com/opencv/opencv_3rdparty/0a0e88972a7ea97708378d0488a65f83e7cc5e69/ffmpeg/ffmpeg_version.cmake... OK