feat(docker): add single-image multi-stage Docker setup with compose

- Multi-stage build: Node builds the frontend, Go builds a static
  CGO-free binary, runtime is Alpine with CA certificates
- Add docker-compose.yml with persistent volume for the SQLite database
- Add .dockerignore to keep the build context slim
- Embed time/tzdata in the Go binary for OS-independent timezones
- Document image build, startup, secrets, and reverse-proxy setup in README

.dockerignore

@@ -0,0 +1,13 @@
+# Build-Kontext schlank halten
+**/node_modules
+frontend/dist
+**/*.db
+**/*.db-wal
+**/*.db-shm
+.env
+**/.env
+!**/.env.example
+.git
+.gitignore
+*.md
+tmp