Your pipelines.yml is mounted into /etc/logstash but the image doesn't look for it there. The /etc/logstash directory is used by the RPM and DEB distributions, but the Docker image is based on the distro-agnostic tarball distribution that keeps all files under the same root. The correct place to mount pipelines.yml in the Docker image is at /usr/share/logstash/config/pipelines.yml. Logstash should find it if you mount it there.
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