Only srt and vtt files are supported currently
nix run github:imakira/subtitle-merger -- -o <output_name.[srt | vtt]> input1.srt input2.vtt ...
The input files need to be under current directory, or whatever you passed on the mounting option.
docker run -t --rm -v ./:/dir coruscation/subtitle-merger -o /dir/out.vtt /dir/test1.vtt /dir/test1.srt