How to make OSX application that just runs opens some file type and runs arbitrary Ruby code?

Programmer Question

It's trivial to make a program executable from shell - just put #!/usr/bin/ruby on top, chmod +x it and done. Unfortunately OSX won't let me associate file type with such scripts - it requires its .apps instead. This sort of distinction doesn't seem to exist on other operating systems.

What's the simplest way of making such .app, which would merely execute some arbitrary Ruby code?

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