Say, you are producing and managing a live stream show, and having guests 'call in' for a performance, but they are more familiar with just streaming themselves to twitch or some other provider. If you want to keep it simple for them, you can stand up this cloned repo with a couple edits, and enable them to RTMP Push directly to you, while you can then RTMP Pull from that running docker container. It uses a slightly older version of nginx with RTMP module, and ffmpeg to do some resolution transcoding. It also has an endpoint to support pushing incoming RTMP out as HLS. This RTMP Pull can be brought into vMix or OBS with the right steps.

There may be an easier way to do this, but this worked for my time constraints.