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I'm afraid that, as of December 2015, the only public luggage lockers in the city Marrakech can be found:
The other option is to ask your hotel to keep your luggage after you check-in. If you are planning on visiting the Jardin before checking-in I will also suggest that you show up to your hotel and ask them to store the luggage for you.
Now I don't know if you can enter the Jardin with a suitcase. You can always try.