Where is Mosquitto and zigbee2mqtt running? These are not part of openHAB. They do not exist in the Docker container. You’ll have to deploy these separately.
Mosquitto and zigbee2mqtt still run on the raspberry the openhabian. This both components still work fine. Only the connection to the new docker does not work.
Did you change the address to Mosquitto on the MQTT Broker Thing? Before you likely used localhost and now you need to use the address of the machine where Mosquitto is running.
Well, then there is something wrong with the networking.
This error means that OH attempted to connect to port 1883 on what ever IP address is configured in the MQTT Broker Thing and there is nothing running on that IP address listening for connections on that port.
So either:
the IP address is wrong and Mosquitto is running on a different IP
the port is wrong and Mosquitto is listening to a different port
Mosquitto isn’t running at all
Mosquitto is in some error state and not able to accept incoming connections
Anything else would generate a different error (e.g. connection timed out)