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.

devbox.json

@@ -1,10 +1,10 @@
 {
   "$schema": "https://raw.githubusercontent.com/jetify-com/devbox/0.10.5/.schema/devbox.schema.json",
   "packages": [
-    "hugo@latest",
+    "hugo@0.151.0",
     "dart-sass@latest",
-    "go@latest",
-    "nodejs@latest",
+    "go@1.25.1",
+    "nodejs@24.10.0",
     "htmltest@latest",
     "go-task@latest"
   ],