Use this docker repository.
Dockerfile that installs ffmpeg on alpine edge.
Hosted at:
https://hub.docker.com/r/kristianp/alpine-ffmpeg
Which is automatically built from this repository.
Starts in the folder /app. Set up a volume to link your video files to the container's /app folder, e.g.
docker run --rm -i -t --name ffmpeg -v "/c/Users/%username%/vid:/app" kristianp/alpine-ffmpeg
The above command starts an interactive shell for exploring the ffmpeg commands available.