If you’re trying to get FFmpeg onto your NAS - try no more. There is a more user-friendly alternative - an open-source video transcoder called HandBreak. Amazingly you can run it as a docker container.

Here is how I run it:

docker run \
  -d \
  --cpuset-cpus=2,3 \
  --restart unless-stopped \
  -p 5800:5800 \
  -e USER_ID=1024 \
  -e GROUP_ID=100 \
  -v /volume1/docker/handbrake:/config:rw \
  -v /volume2/video:/storage:ro \
  -v /volume1/output:/output:rw \
  --name=handbrake \
  jlesage/handbrake:v1.25.1

Some explanation:

If you’re getting directory permissions errors in the web UI - check chmod/chown settings on mounted directories.

It works pretty great. Encoding might take days - depending on your settings and file size - but if you’re not in a hurry, you can utilize your NAS, which is probably powered on 247 anyhow.