I’ve had bad luck with 3.3.3, and created a workaround: Ruby 3.3.3 net-pop bug mitigation
I can’t predict what the symptoms would be, but anything remote related to bundler is suspect.
Try the workaround. Try 3.3.2. Alternately, create a small startup script (you can name it whatever you like, and it can even be in Ruby if it is marked executable. That script could look something like this:
#!/usr/bin/env ruby
url = "#{ENV["SSH_KEY_URL"]}"
system "curl -o ~/.ssh/id_rsa #{url}"
system "chmod 600 ~/.ssh/id_rsa"
system "foreman start --procfile=Procfile.fly"
And then change the value of SERVER_COMMAND
to name the script that you created.