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Firstly, here's an excellent video demo (from Information Insights) showing Universal Connector end to end for MongoDB Atlas:
https://www.youtube.com/watch?v=LAYhVoYMb28You need a method to get the events out of MongoDB and into the Logstash pipeline. The video demo shows how you can save the events to files and then push them to the Logstash pipeline (Universal Connector). This might work for your POC. You could mount a shared folder to the docker container (dockerfile ADD) and in this way, have access to the events/files outside the container and then push the events (using Filebeat) to the Universal Connector.
I hope this makes sense.