[docker_image_ctl.j2] skip hostname update if is up to date (#3529)

Signed-off-by: Stepan Blyschak <stepanb@mellanox.com>

files/build_templates/docker_image_ctl.j2

@@ -185,7 +185,10 @@ start() {
             {%- endif %}
             preStartAction
             docker start {{docker_container_name}}
-            updateHostName "$HOSTNAME"
+            CURRENT_HOSTNAME="$(docker exec {{docker_container_name}} hostname)"
+            if [ x"$HOSTNAME" != x"$CURRENT_HOSTNAME" ]; then
+                updateHostName "$HOSTNAME"
+            fi
             postStartAction
             exit $?
         fi