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After starting TeamCity with the JetBrains docker-compose sample, no build agents appeared in the server UI even though the agent containers were running fine. The default buildAgent.properties points the agent at the local server on port 8111, so each agent tried to reach a TeamCity server on itself instead of the actual server container. Changing the URL to the host IP or the published port did not help either.

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