The explanation for that gnarly template string can be found at www.rsyslog.com/doc/configur...

Minor change to the rsyslog config, in modern syntax:
-----
template(name="MyLog"
type="string"
string="/var/log/mylog/%programname:F,45:2:%.log")

if ($programname startswith 'docker-') then {
action(type="omfile" dynaFile="MyLog")
stop
}
-----