use unicorn from git with custom build steps for Ruby 4

[catatsuy]

This pull request updates the Ruby Docker build and the Gemfile to support building and running the unicorn gem from a specific Git commit, rather than from RubyGems, and ensures its native extension is properly built inside the Docker image. It also upgrades the Ruby base image to version 4.0-slim and adds additional build dependencies.

Dependency and build process updates:

• Upgraded the Docker base image from ruby:3.4-slim to ruby:4.0-slim in webapp/ruby/Dockerfile.
• Added git and ragel to the list of system packages installed in the Docker image to support building the unicorn gem and its native extensions.

Unicorn gem customization:

• Changed the Gemfile to install unicorn directly from a specific commit on GitHub instead of RubyGems, locking to commit e9862718a7e98d3cbec74fc92ffc17a1023e18da.
• Updated the Docker build process to:
  • Locate the vendored unicorn gemspec,
  • Generate and write the unicorn/version.rb file with the correct version,
  • Build the native extension using ragel and make,
  • Copy the resulting unicorn_http.so to the correct location in the gem directory, ensuring the extension is available at runtime.

[catatsuy]

refs: https://yhbt.net/unicorn-public/[email protected]/T/#u