gentoo-syslog/Dockerfile.template: Tweak sources (-system +socket)
Don't listen to /proc/kmsg, because Docker doesn't mount it for us.
This avoids:
* Starting syslog-ng ...
Error opening file for reading;
filename='/proc/kmsg', error='Operation not permitted (1)'
Error initializing message pipeline;
* start-stop-daemon: failed to start `/usr/sbin/syslog-ng'
* Failed to start syslog-ng [ !! ]
* ERROR: syslog-ng failed to start
Also add an explicit source listening to /dev/log. I don't have this
on my host system, and that syslog-ng creates /dev/log just fine, but
my containers seem to need an explicit listing here or they don't
create /dev/log.
Reported-by: Oleg Terenchuk <litwol@litwol.com>