Use Debian's packaged Ruby instead of compiling from source

[JamieMagee]

Problem

The Dockerfile currently compiles Ruby 3.2.2 from source using ruby-build. This takes about 5 minutes and accounts for most of the Docker build time.

curl -L https://github.com/rbenv/ruby-build/archive/refs/tags/v20231012.tar.gz | tar -zxvf - -C /tmp/ && \
cd /tmp/ruby-build-* && ./install.sh && cd / && \
ruby-build -v 3.2.2 /usr/local && rm -rfv /tmp/ruby-build-*

Why we don't need to do this

Ruby is only used to run Licensee (the license detection tool). Licensee 9.16.1 requires Ruby >= 3.0.

Debian Trixie (our base image node:24-trixie) ships with Ruby 3.3, which satisfies this requirement.

Proposed change

Replace the ruby-build compilation with:

apt-get install -y ruby ruby-dev

Build time comparison

Build	Time
Before (compile from source)	6m 28s
After (Debian package)	4m 5s
Saved	2m 23s (37%)

The Ruby installation step specifically went from ~5 minutes to ~14 seconds.