Insufficient clarity around setting environment variables at build time

[zxaos]

There's some writing about how to do this for rails but it may not be sufficient.

Feedback from someone trying out the service:

I was just trying out Fly for the first time and I have failed HARD

my use case is that I have an app that needs to access env vars during the build, and when I get to https://fly.io/docs/reference/build-secrets/ this page makes zero sense to me — I don't know anything about Docker and I thought the reason to use Fly was that things would Just Work™

See also discussion in slack: https://flyio.slack.com/archives/CPGPG7A5C/p1681748552524199