Allow 'lack of build target'

**Context**

A project I contribute to is in quite a unique position - it needs to reference UnityEditor assemblies (and some unity package assemblies), and then be built outside of unity.
To this end, we use the unity-builder action to get access to the necessary assemblies from GitHub actions.
It's a bit frustrating that we have to choose a 'target platform' when we actually don't want to build anything at all.

All we need to do is open a project (to restore packages), copy some files, and close the project again.

**Suggested solution**

Provide a 'none' build target that uses the `base` docker images.