Graylog 4.2.8 in a docker, plugin issue

[Laro88]

I am trying to use the plugin 1.1.1 inside docker, it loads and seems configurable, however I can't establish a connection to either local or external mqtt servers. The mosquitto:1883 is the compose name of the service, I have tried tcp:// variants, external ip numbers etc. without any luck.
Any ideas - or is this a dead end?

 2022-04-29 08:56:20,342 ERROR: xenqtt - Failed to connect a client MQTT channel to mosquitto:1883
graylog-graylog-1        | java.nio.channels.UnresolvedAddressException: null
graylog-graylog-1        |      at sun.nio.ch.Net.checkAddress(Net.java:104) ~[?:1.8.0_322]
graylog-graylog-1        |      at sun.nio.ch.SocketChannelImpl.connect(SocketChannelImpl.java:621) ~[?:1.8.0_322]
graylog-graylog-1        |      at net.sf.xenqtt.message.AbstractMqttChannel.<init>(AbstractMqttChannel.java:105) [graylog-plugin-mqtt-1.1.1.jar:?]
graylog-graylog-1        |      at net.sf.xenqtt.message.MqttClientChannel.<init>(MqttClientChannel.java:43) [graylog-plugin-mqtt-1.1.1.jar:?]
graylog-graylog-1        |      at net.sf.xenqtt.message.ChannelManagerImpl$NewClientChannelCommand.doExecute(ChannelManagerImpl.java:617) [graylog-plugin-mqtt-1.1.1.jar:?]
graylog-graylog-1        |      at net.sf.xenqtt.message.AbstractBlockingCommand.execute(AbstractBlockingCommand.java:87) [graylog-plugin-mqtt-1.1.1.jar:?]
graylog-graylog-1        |      at net.sf.xenqtt.message.ChannelManagerImpl.executeCommands(ChannelManagerImpl.java:441) [graylog-plugin-mqtt-1.1.1.jar:?]
graylog-graylog-1        |      at net.sf.xenqtt.message.ChannelManagerImpl.doIO(ChannelManagerImpl.java:309) [graylog-plugin-mqtt-1.1.1.jar:?]
graylog-graylog-1        |      at net.sf.xenqtt.message.ChannelManagerImpl.access$000(ChannelManagerImpl.java:47) [graylog-plugin-mqtt-1.1.1.jar:?]
graylog-graylog-1        |      at net.sf.xenqtt.message.ChannelManagerImpl$1.run(ChannelManagerImpl.java:92) [graylog-plugin-mqtt-1.1.1.jar:?]
graylog-graylog-1        |      at java.lang.Thread.run(Thread.java:750) [?:1.8.0_322]
graylog-graylog-1        | 2022-04-29 08:56:20,343 ERROR: org.graylog2.shared.inputs.InputLauncher - The [org.graylog2.inputs.mqtt.MQTTGELFInput] input with ID <626ba3c0bc979d2d7d6e8828> misfired. Reason: Command failed: NewClientChannelCommand; ROOT CAUSE: null.