How to change the log level with a Docker container?

[Ipheion]

Continuing the discussion from Having problems with UrBackup? Please read before posting:

@uroni

The server log file is per default located at C:\Program files\UrBackupServer\urbackup.log. Change warn to debug in C:\Program files\UrBackupServer\args.txt to create a debug log file. Debug log files are preferred. The log file on Linux/FreeBSD is located per default at /var/log/urbackup.log. The log level is an argument to the server program (urbackupsrv run -v debug) or can be configured via /etc/default/urbackupsrv or /etc/sysconfig/urbackupsrv.

I am running my UrBackup server in a Docker container. How do I change the log level with that?