Query Fly for all running instances by app name from within the app

[ignoramous]

@kentcdodds

To do this I need the instance hostname.

1. Retrieve alloc-ids of an app: Feature request: list of running ALLOC_IDs via DNS - #2 by charsleysa
2. Address them: Fly Apps on machines: prerelease of fly deploy - #8 by kurt

Docs: Private Networking · Fly Docs