Allow injection of sources json as an environment variable directly

[Mystik738]

Rather than require custom sources.json files to build new images to use (and therefore require users to manage their own images), you should allow JSON to be injected directly as configuration through an environment variable, which, along with volumes, would enable the one image to handle arbitrary configuration.

[Mystik738]

As an example, I'm handing this in my own repo here: Mystik738@c7c7bc5 but this change would effectively wipe out the need for creating a custom container image and therefore would have a larger documentation change.

I wouldn't mind doing that work, but only if this is an approved feature.