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.

.env.versions

@@ -0,0 +1,9 @@
+# Tool versions for development and CI/CD
+# These versions are used in:
+# - devbox.json (pinned versions)
+# - Dockerfile (build arguments)
+# - .github/workflows/ci.yaml (CI/CD pipeline)
+
+NODE_VERSION=24.10.0
+GO_VERSION=1.25.1
+HUGO_VERSION=0.151.0