In this CI job, the oxidize-rb/actions/cross-gem@v1 action runs:

 rb-sys-dock --platform x86_64-linux-gnu --directory . --ruby-versions 3.1,3.2,3.3,3.4 --build

This now correctly pulls down the image rbsys/x86_64-linux-gnu:0.9.109, which is an alias for rbsys/x86_64-linux:0.9.109.

However, the RUBY_TARGET is set here:

This should actually be x86_64-linux-gnu, not x86_64-linux. As a result, the build ends in failure:

Don't know how to build task 'native:x86_64-linux' (See the list of available tasks with `rake --tasks`)
Did you mean?  native:x86_64-linux-gnu
/tmp/rb-sys-dock/bundle/ruby/3.4.0/gems/rake-13.2.1/exe/rake:27:in '<top (required)>'
/usr/local/rbenv/versions/3.4.1/bin/bundle:25:in 'Kernel#load'
/usr/local/rbenv/versions/3.4.1/bin/bundle:25:in '<main>'

This distinction is important because gems compiled with x86_64-linux can work both on GNU and musl systems, but x86_64-linux-gnu is intended for GNU systems.

This probably can be solved by:

1. Changing RUBY_TARGET from an ENV to an ARG.
2. Having rb-sys-dock pass in -e RUBY_TARGET.