transparent-gateway-details.rst: add docker install info

Fixes eclipse/oniro-core/docs#117 (closed)

Signed-off-by: Grzegorz Gwóźdź grzegorz.gwozdz@huawei.com

transparent-gateway-details.rst

@@ -18,7 +18,8 @@ In this blueprint, the Makerdiary nrf52840 MDK USB dongle is used to provide the
 
 The current manual process is quite intricated, but expect vast improvements in future iterations of the blueprint.
 
-- Setup the OpenThread environment as follows:
+- Make sure you have `docker` installed on your computer. 
+  If so, setup the OpenThread environment as follows:
 
 .. code-block:: bash