Ok
Then i will start first with installing an Docker image from Mosquitto on the NAS and configuring existing EMONCMS for the external MQTT server. Found some infos in the forum how this should work.
As my prefered solution is to have a seperate docker for EMONCMS on the NAS and a seperate Docker for MQTT also on the NAS which i can use for all the needed IOT staff for my homeautomatisation i´m working on (openHAB2+LCN+VELUX+SQUEEZE). As i planning to integrate some shelly´s which use also MQTT.