Update Dockerfile for caching layers and change CMD to ENTRYPOINT

[catatsuy]

This pull request includes several updates to the webapp/ruby/Dockerfile to improve Docker image build efficiency and ensure consistency in the application startup process.

Dockerfile Improvements:

• Added Dockerfile syntax version declaration at the top of the file (#syntax=docker/dockerfile:1).
• Implemented build cache mounts to speed up apt-get update and package installation (--mount=type=cache,target=/var/lib/apt,sharing=locked and --mount=type=cache,target=/var/cache/apt,sharing=locked).
• Changed the way Gemfile and Gemfile.lock are copied to the Docker image to ensure they are in the correct directory before running bundle install.
• Replaced CMD with ENTRYPOINT to ensure the application always starts with the specified command (bundle exec rackup -p 8000 -o 0.0.0.0).