feat(ci): add Docker build pipeline with version management

- Add multi-stage Dockerfile with pinned tool versions (Node 24.10.0, Go 1.25.1, Hugo 0.151.0)
- Create .env.versions as single source of truth for all tool versions
- Add GitHub Actions CI workflow for automated OCI image builds
  - Multi-arch support (amd64, arm64)
  - Automatic version loading from .env.versions
  - Docker registry push with metadata tags
- Add Taskfile tasks for local OCI image building and testing
  - task build:oci-image - Build with version-pinned dependencies
  - task test:oci-image - Build and test container locally
- Pin devbox.json to specific versions matching .env.versions
- Add comprehensive documentation (DOCKER.md, VERSIONS.md)
- Add helper script (scripts/get-versions.sh) for version extraction

This enables consistent development and production environments with
identical tool versions across local devbox, Docker builds, and CI/CD.

.dockerignore

@@ -0,0 +1,19 @@
+.github
+.gitignore
+.vscode
+.devbox
+.devcontainer
+node_modules
+public
+resources
+tmp
+*.md
+!content/**/*.md
+TESTING.md
+Taskfile.yml
+devbox.json
+devbox.lock
+.hugo_build.lock
+.htmltest.yml
+.htmlvalidate.json
+.markdownlint.json