expose host dir /host/reboot-cause to pmon docker so that the reboot causing by user command can be identified

files/build_templates/docker_image_ctl.j2

@@ -191,6 +191,9 @@ start() {
 {%- if docker_container_name != "database" %}
         -v /usr/share/sonic/device/$PLATFORM/$HWSKU:/usr/share/sonic/hwsku:ro \
 {%- endif %}
+{%- if docker_container_name == "pmon" %}
+        -v /host/reboot-cause:/host/reboot-cause \
+{%- endif %}
 {%- if sonic_asic_platform != "mellanox" %}
         --tmpfs /tmp \
 {%- endif %}