The container CMD sets the -e(See CMD at Dockerfile.j2) flag by default which logs to stderr and disables syslog/file output.
This means configuring logging.to_files in filebeat.yml has no effect

• Beat Version: 6.3.1
• Operating System: MacOS High sierra 10.13.5
• Docker Version: Version 18.03.1-ce-mac65 (24312)
• Steps to Reproduce: Add logging.to_files in filebeat.yml.
  https://www.elastic.co/guide/en/beats/filebeat/current/configuration-logging.html