Inside the Restreamer Docker container is a RTMP-Server running (nginx-rtmp) that you can use to push streams to. To have access to the RTMP-Server, you have to expose the port 1935 of the Docker container, e.g.
docker run ... -p 1935:1935 ... datarhei/restreamer
This is inheritly dangerous because anyone can now push RTMP streams to your box. In order to protect the RTMP-Server you should define a token (more details), e.g.
docker run ... -p 1935:1935 -e RS_TOKEN=a-secret-only-you-know ... datarhei/restreamer
Now you can push a RTMP-Stream to, e.g.
rtmp://[address of your box]/live/external.stream?token=....
Navigate to the Restreamer web interface, e.g.
http://[address of your box]:8080/. For the video source you enter the same address
you are pushing to, and replace the public address and port with localhost, i.e.
Now you can forward it to an other RTMP-Server or embed the player in a website.
With Restreamer you can forward your stream to an external video streaming provider like YouTube.