@J_V, for info, there is a draft RC4 that implements this: you might give a try and give us feedback.
docker pull pydio/cells:2.2.0-rc4
We are currently working on the documentation for the major simplifications that are introduced with 2.2 release, so you are a bit on your own for now, but main tip is: CELLS_BIND env variable takes precedence over sites that are stored in the local config (a.k.a: the pydio.json)
GA should come within a week or two
Please let us know what you think. Thanks!