I have a DS713+ that supports Docker and needed a way to install an iPerf3 server for WLAN testing. Fortunately, that is pretty easy. Activate SSH on your Synology so you can remote in. Using PuTTY, login as an admin user and type the following: sudo docker pull networkstatic/iperf3 (then type your admin password again)
My collection of Home Assistant, CoreELEC, IoT, Synology, Kodi and other technical tips gathered over the years.