ruby-dev 1:2.5~1000grau (armhf binary) in ubuntu bionic

 Ruby is the interpreted scripting language for quick and easy
 object-oriented programming. It has many features to process text
 files and to do system management tasks (as in perl). It is simple,
 straight-forward, and extensible.
 .
 This package contains the header files and the mkmf library, necessary
 to make extension library for Ruby. It is also required to build
 many gems.
 .
 This package is a dependency package, which depends on Debian's default Ruby
 version (currently v2.5).