Test OpenCV build with various FFmpeg libraries.

Assuming following folder layout:

- opencv/
- videoio-ffmpeg-check/ (this project)
- - workspace/
- - ffmpeg/
- - scripts/
- - - build.sh
- - run.sh
- - download.sh

Instruction (run in current directory):

• ./download.sh - download selected FFmpeg source archives to ffmpeg/ directory
• ./run.sh - build and run Docker container
• (in container) /scripts/build.sh - build each FFmpeg and OpenCV