feat(01-01): create multi-stage Dockerfile

- Multi-stage build using golang:1.25-bookworm and debian:bookworm-slim
- CGO_ENABLED=0 for static binary compilation
- Cross-platform support via TARGETOS/TARGETARCH
- Non-root user (appuser) for security
- Data volume mount at /data
- Port 32768 exposed
- .dockerignore excludes build artifacts and planning docs

.dockerignore

@@ -0,0 +1,11 @@
+.git
+.gitignore
+README.md
+*.md
+.env
+.env.local
+.DS_Store
+.air.toml
+docker-compose.yml
+.planning/
+tmp/