Inline base image into Dockerfile to fix Node 20 requirement

[graygilmore]

Summary

Fixes #103. Shopify CLI 3.93+ requires Node >=20.10.0 (engine-strict: true), which breaks the action since the base Docker image used Node 18.

Rather than maintaining a separate base image on a container registry (which requires org-level permissions we couldn't obtain for ghcr.io), this inlines everything into a single Dockerfile.

Changes

• Dockerfile now includes all dependencies (Node 20 Bookworm, Chrome, Puppeteer, LHCI, Shopify CLI)
• Removed Ruby (rbenv, ruby-build) — the current Node.js-based Shopify CLI doesn't need it
• Removed Dockerfile.base and build-base-image.yml workflow
• Simplified Makefile for local development

Performance

GitHub Actions caches Docker layers per-repo, so:

• First run for any user of the action: faster now without Ruby compilation
• Subsequent runs: Fast (layers cached)

This avoids the need for any container registry (ghcr.io, Docker Hub) entirely.

Supersedes

This replaces the approach in #105 and #106 which attempted to use ghcr.io.

🎩 Tophatting

This is infrastructure-only (Docker build), so CI passing is the primary verification.

1. Confirm CI passes (the check_dawn jobs exercise the action end-to-end)
2. Optionally, build locally to verify the Dockerfile works:

   make build

3. Optionally, shell into the container to inspect the environment:

   make ssh
   node --version   # Should show v20.x
   shopify version  # Should show 3.93+