RAILS_MASTER_KEY environment variable not getting fetched

[ignoramous]

Build-time secrets are different from runtime secrets: Build-time secrets - #2 by fideloper-fly

See this for one solution: `fly deploy --remote-only` isn't reading Rails Credentials file from Github Actions, but works elsewhere - #3 by jsierles

Or, this for another: Secret docker build arguments - #5 by jsierles