[docker-mender-convert] Echo the path of the logfile at the end

The suffix is random, so this will help the user to inspect the correct
log file.

Changelog: None

Signed-off-by: Lluis Campos <lluis.campos@northern.tech>

docker-mender-convert

@@ -45,3 +45,5 @@ docker run \
   $IMAGE_NAME "$@"
 
 [ $? -eq 0 ] && rmdir ${WORK_DIR}
+
+echo "Log file available at: logs/${LOG_FILE}"