This solved it for me:
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: falseto the Filebeat configuration. So, not a bug per se in the OSS Docker build.