I created a custom Dockerfile to add ffmpeg to the container. Version can be any version, I used 4.0.1.
Dockerfile:
FROM openhab/openhab:4.0.1
RUN apt-get update
RUN apt-get install ffmpeg -y
docker build -t openhab/openhab4-custom .
After the image is build you can use it with a docker-compose file.