Mysterious Filebeat 7 X-Pack issue using Docker image
It turns out that starting in one of the 7.x versions they turned on index lifecycle management checks by default. ILM (index lifecycle management) is an X-Pack feature, so turning this on by default means that Filebeat will do an X-Pack check by default.
This can be fixed by adding setup.ilm.enabled: false to the Filebeat configuration. So, not a bug per se in the OSS Docker build.