ERROR [stage-4 6/6] RUN bin/rails fly:build when 'fly deploy'

That does look like the same error. miru-web is a rails 7 application. As such, it should not have rails/webpacker, rails/ujs, or turbolinks in package.json, and should have a build script. @Ouska reported that making these changes enabled him to deploy.

Since your app is open source, if the above doesn’t work for you let me know how to reproduce the problem and I’ll try to reproduce the problem.