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Inside the compose bridge network, agents must address the server by its service alias and internal port, not the loopback address or the host's published port. In each agent's buildAgent.properties set serverUrl to point at the teamcity service on its internal port 8111 (backslash-escaped per the properties format), keep the server's internal port 8111, and leave the 8112:8111 host mapping for browser access. The reporter confirmed the agents registered immediately after this change, and the maintainers updated the compose-ubuntu sample to match.
Source: https://github.com/JetBrains/teamcity-docker-samples/issues/4