Support the use of a config file like binder.json?

[cboettig]

Would it be possible to make binder deploy configurable with some kind of config binder.json file? My understanding is that it currently just relies on conventions to decide where to find requirements.txt or a Dockerfile and what to do. Having common conventions is great and all, but also constraining. e.g. a simple config might just say: "use this Docker image" (e.g. a Dockerfile in a hidden folder, or a Docker image on another repo), or point to where to find a requirements.txt file to add to the default image? a somewhat richer set of config options could say "run this command after start", allowing for more general customization without touching the docker layer directly.