console-getty.service: don't start when /dev/console is missing

Create minimal image which runs systemd

   FROM rhel7.1
   RUN yum install -y /usr/bin/ps
   ENV container docker
   CMD [ "/usr/sbin/init" ]

When you run the container without -t, the process

   /sbin/agetty --noclear --keep-baud console 115200 38400 9600

is not happy and checking the journal in the container, there is a stream of

Mar 13 04:50:15 11bf07f59fff agetty[66]: /dev/console: No such file or directory
Mar 13 04:50:25 11bf07f59fff systemd[1]: console-getty.service holdoff time over, scheduling restart.
Mar 13 04:50:25 11bf07f59fff systemd[1]: Stopping Console Getty...
Mar 13 04:50:25 11bf07f59fff systemd[1]: Starting Console Getty...
Mar 13 04:50:25 11bf07f59fff systemd[1]: Started Console Getty.
Mar 13 04:50:25 11bf07f59fff agetty[67]: /dev/console: No such file or directory
Mar 13 04:50:35 11bf07f59fff systemd[1]: console-getty.service holdoff time over, scheduling restart.
Mar 13 04:50:35 11bf07f59fff systemd[1]: Stopping Console Getty...
Mar 13 04:50:35 11bf07f59fff systemd[1]: Starting Console Getty...
Mar 13 04:50:35 11bf07f59fff systemd[1]: Started Console Getty.
Mar 13 04:50:35 11bf07f59fff agetty[74]: /dev/console: No such file or directory
Mar 13 04:50:45 11bf07f59fff systemd[1]: console-getty.service holdoff time over, scheduling restart.
Mar 13 04:50:45 11bf07f59fff systemd[1]: Stopping Console Getty...
Mar 13 04:50:45 11bf07f59fff systemd[1]: Starting Console Getty...

units/console-getty.service.m4.in

@@ -9,6 +9,7 @@
 Description=Console Getty
 Documentation=man:agetty(8)
 After=systemd-user-sessions.service plymouth-quit-wait.service
+ConditionPathExists=/dev/console
 m4_ifdef(`HAVE_SYSV_COMPAT',
 After=rc-local.service
 )m4_dnl