I seem to be having the same issue, and cannot for the life of me figure it out.
Secrets are set, nearly identical config to @m.fahle, and yet the application simply doesn’t come up. Nothing in the logs, and the HTTP health check fails.
I followed the guide in the Fly docs, but I did also update to Elixir 1.12.1 and Erlang 24.0.1 with the adjustments from this comment Elixir Getting Started Guide - #16 by zimt28