This is called a staging environment.

I can’t find any concrete references to back this up, it’s a term I often use to describe a’sandbox’ website that is under development. I think it really calls on several uses of the word;

from TFD

Staging – any temporary structures used in the process of building, esp the horizontal platforms supported by scaffolding

Whilst I appreciate this definition refers to physical stages and platforms, you are essentially putting in the support structures for software, rather than a physical building.

from TFD

Staging Area – Any place serving as a point of assembly or preparation.

As used by civilians to describe location of troops and equipment.

What is our staging environment in which things are built and prepared, before deployment?