dockerfile-bundle-install-without-cache-mount

Detects Ruby Dockerfiles that run bundle install without a visible BuildKit cache mount on the same instruction.

This rule looks for:

• a Docker build discovered from GitHub Actions
• Gemfile in the build context
• RUN bundle install
• no --mount=type=cache on that Dockerfile instruction

Why it matters:

• Bundler installs can download gems and compile native extensions.
• Without BuildKit cache mounts for Bundler caches, Docker rebuilds can repeatedly pay gem download and installation costs.
• Depot’s optimized Ruby Dockerfile mounts Bundler’s internal cache and the vendor cache during gem installation.

What to do:

• Add a BuildKit cache mount for /usr/local/bundle/cache to the bundle install step.
• If the Dockerfile uses bundle cache, also consider mounting /app/vendor/cache.
• Keep Gemfile and Gemfile.lock copied before broader source files when practical.

This rule focuses on Docker build cache behavior, not Rails runtime configuration.