Because the [Synapse](https://github.com/matrix-org/synapse) Matrix server is originally very chatty when it comes to logging, we intentionally reduce its [logging level](https://docs.python.org/3/library/logging.html#logging-levels) from `INFO` to `WARNING`.
If you'd like to debug an issue or [report a Synapse bug](https://github.com/matrix-org/synapse/issues/new/choose) to the developers, it'd be better if you temporarily increasing the logging level to `INFO`.
You can free some disk space from Docker, see [docker system prune](https://docs.docker.com/engine/reference/commandline/system_prune/) for more information.