move logs to a log file

[rjnn]

stdout is super crowded

[quodlibetor]

This should probably be done by way of actually using log4j and allowing users to specify a custom config. It might be nice to have a --log-file FNAME, but doing that requires thinking about log retention. Spewing to stdout has the benefit that we offload that responsibility to docker/systemd in general.

[3noch]

At least keep the option around for stdout for the reason @quodlibetor mentioned. I usually just rely on systemd journal to handle retention etc.