Populate secret and/or config stores from environment variables

[joeshaw]

You can populate config or secret stores either inline in the fastly.toml file or by reading a file from disk. It would be nice if you could also populate them with environment variables from the host environment.

My current use case for this is passing in an API credential when running Viceroy in a test runner mode. I don't want to check this credential into git, and I would rather not have to bother with an external, local-only file.