Install Networkstatic/iperf3 within Docker to port
do not start iperf3 from the docker GUI 5201, but execute from CLI to create a temporary container running iperf3 as a server:
sudo docker run -it --rm -p 5201:5201 networkstatic/iperf3 -s
From your a computer, install iperf3 (under Windows, I found the binaries here: https://iperf.fr/iperf-download.php#windows) and run:
iperf3 -c 192.168.x.xxx
Stopping can be done by CLI (ehm docker stop or something), but can also be done on the GUI