Easier file logging

[scionaltera]

The file appender is commented out in the logback configuration. When it is uncommented it produces a log file inside the docker container at /tmp/spring.log. It would be nice to configure the filename of the log file to arbitrader.log and provide a volume mapping for the file so that when you run docker-compose up it would put arbitrader.log in the current working directory outside the container.

Logging to a file should be off by default, but easy to enable if desired.