add missing ruby 3 variants

[casperdcl]

• fixes missing ruby:*-3* variants #1906

Note that both ruby 3 & 4 are meant to be supported at least until Mar 2028 so this PR should be merged.

[Copilot]

Pull request overview

Fixes the published tag mapping for the Ruby 3 line by ensuring the :3* aliases resolve to the Ruby 3.4 variants (instead of incorrectly resolving to Ruby 4 for :3-trixie, per #1906).

Changes:

• Add missing ruby:${VERSION}-3 and ruby:${VERSION}-3-<distro> alias tags to the 3.4-* variants so :3, :3-trixie, :3-bookworm, and :3-bullseye resolve correctly.
• Add the ruby:${VERSION}-3.4 alias tag to the 3.4-trixie variant.