Got flow idea : spawn 1 docker instance for each feature branch so the testing team can test each feature in isolation. Good idea or not?

Pretty good idea, I've been in an instance where I've had to pick up somebody else's project and deploying a local setup has been a nightmare because of some dependencies they forgot to mention.
On the other hand it'll mean needing to setup your environment for every feature which might get tedious, although I guess you could have a setup script for that.