Download the code: git clone https://github.com/owncast/owncast
Copy config-default.yaml to config.yaml
Edit config.yaml with a file editor of your choice and change the path of ffmpeg by appending ffmpegPath: /usr/bin/ffmpeg at the top level of the yaml.
If you ever make any future config file changes you must rerun the docker build step otherwise you can just run the docker run step to run the service going forward.
Run docker build -t owncast . and wait. It may take a few minutes to build depending on the speed of your server.
Run docker run -p 8080:8080 -p 1935:1935 -it -v ./config.yaml:/app/config.yaml owncast to start the service with your custom configuration.