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>