I live in a Victorian terrace which has one to access the gardens. I call it the alleyway; but it's slightly different to the one in your pic in that my garden access is open to the sky, rather than arched. I'm not sure if being sky-open makes a difference to the term, but 'alleyway' does sound more like it's between two walls rather than enclosed.
Passageway, with some description, seems more likely, though does have unfortunate connotations.
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