[supervisord]: use abspath as supervisord entrypoint (#5995)

use abspath makes the entrypoint not affected by PATH env.

Signed-off-by: Guohan Lu <lguohan@gmail.com>

dockers/docker-dhcp-relay/docker_init.sh

@@ -19,6 +19,6 @@ sonic-cfggen $CFGGEN_PARAMS
 chmod +x /usr/bin/wait_for_intf.sh
 
 # The docker container should start this script as PID 1, so now that supervisord is
-# properly configured, we exec supervisord so that it runs as PID 1 for the
+# properly configured, we exec /usr/local/bin/supervisord so that it runs as PID 1 for the
 # duration of the container's lifetime
-exec supervisord
+exec /usr/local/bin/supervisord