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.